Heat Transfer Characteristics of Passive, Active, and Hybrid Impinging Jets: A Review

In engineering applications, there is a need to reform the excitation techniques used for jet impingement to achieve simpler, more effective thermal exchange. Jet impingement is a highly effective method for enhancing heat transfer due to its ability to create high heat transfer coefficients. This makes it a preferred technique in applications requiring efficient thermal management, such as cooling of electronic components, turbine blade cooling in jet engines, and material processing. However, traditional excitation techniques for jet impingement can be complex and challenging to implement. Therefore, there is a growing interest in developing new excitation methods that are simpler and more effective. The employed techniques can be categorized into three groups: passive self-excited jets, active excited jets, and hybrid techniques. Passive methods, such as annular, swirling, and sweeping jets, utilize the inherent characteristics of the jet flow without requiring additional energy consumption. Active systems, on the other hand, involve supplementary devices like fans or pumps to intensify heat transfer. Examples of active excited jets include synthetic and pulsed jets. Hybrid techniques combine two or more methods to further thermal improvement. This paper reviews experimental and numerical hybrid techniques to enhance heat transfer to impinged surfaces. Experimental tools, including high-speed imaging, Particle Image Velocimetry (PIV), and infrared thermography, are shown. Numerical simulation methods, such as Computational Fluid Dynamics (CFD), are reviewed. The efficacy of these methods is evaluated by comparing their performance, highlighting potential for optimization and innovation in jet impingement heat transfer.

Heat Transfer, Hybrid Techniques, Impinging Jets, Flow Dynamics

Efficient thermal management is an important aspect of various engineering applications, ranging from electronics cooling to industrial processes. A very efficient method for enhancing heat transfer is jet impingement, which involves directing a jet of fluid onto a target surface. Many studies were performed numerically and experimentally with different methods to highlight heat transfer and flow dynamics interactions. The efficiency of this method largely depends on the excitation method employed to generate and control jet flow (El Hasan et al., 2024; Zohbi et al., 2024; Assoum et al., 2023; Aldossary et al., 2022; Susmiati et al., 2022; Mrach et al., 2020; Hassan et al., 2020; Assoum et al., 2020; Jani et al., 2019; Zhang 2009).

1. Simple Jets

   
   

   Over the past few decades, numerous experimental and numerical studies have focused on enhancing heat transfer to cool heated surfaces using impinging jets. The interest in this area stems from the critical need to improve thermal management systems across various industrial applications. Researchers have identified several key parameters that significantly influence heat transfer performance. These parameters include the composition of the fluid, the material and geometry of the plate, the shape and geometry of the nozzle, the impact distance between the nozzle and the surface, and the role of resonance effects. Each of these factors contributes uniquely to the efficiency of heat transfer, making them vital considerations for optimizing impinging jet systems (Matar et al., 2024).

   1. Fluid Composition

   The thermal properties of the working fluid play a pivotal role in heat transfer performance. Fluids with higher thermal conductivity, such as water-based nanofluids, have been shown to enhance heat dissipation. The addition of Al?O? at a concentration range between 0.15% and 0.6% can enhance thermal transfer by up to 128%. Hybrid nanofluids can further improve the fluid's ability to transfer heat by mixing additional nanoparticles, which increase turbulence and thermal conductivity. Additionally, the specific heat capacity and viscosity of the fluid influence its flow behavior and cooling effectiveness (Ranga Babu et al., 2017; Modak et al., 2015).

   2. Plate Modification 

   The material and geometry of the target plate significantly affect the flow dynamics and heat transfer characteristics. Surface roughness can enhance heat transfer by disrupting the boundary layer and promoting turbulence. However, excessive roughness may lead to higher pressure drops and energy losses. Surface modification can be achieved by adding pin fins of different geometries (rectangular, square, circular, and elliptical) to the surface, which impact both the velocity distribution and the magnitude of the backflow area near the pins, thereby enhancing heat transfer. Among these, elliptical fins demonstrate the highest enhancement in Nusselt number, with an increase of up to 54%. On the other hand, surface modification can also be achieved by applying linear or radial microgrooves, which show significant enhancement compared to a flat, smooth plate (Ravanji and Zargarabadi,2021; Jenkins et al., 2017).

   The composition of the plate plays a crucial role in enhancing heat transfer by altering flow dynamics and thermal interactions. Studies have investigated the application of a magneto-fluid coating on hot surfaces, which resulted in a significant enhancement of heat exchange, influenced by jet velocity and reaching up to 32%. This improvement is attributed to the presence of vortices within the ferrofluid. Additionally, the use of porous materials, such as aluminum foam and resin foam on a plain wall, has been explored. These materials were found to increase hydraulic resistance against the incoming jet, as evidenced by the distribution of static pressure along the surface, further contributing to improved heat transfer performance (Yogi et al., 2022; Chen et al., 2001).A flat plate provides a uniform surface for jet impingement, whereas curved plates, as illustrated in Figure 1, create complex flow structures, including secondary vortices, which enhance mixing and significantly improve heat transfer (Kim et al., 2019; Li et al., 2019; Taghinia et al., 2016).

   
   

   

   Figure 1 Concave surface

   Adjusting the inclination angle of the plate redistributes the impingement region, influencing heat transfer performance. Studies have shown that using jets at different wall inclinations enhances heat exchange. This improvement is due to the formation of vortical structures, which disrupt the laminar flow pattern and promote more effective heat transfer (Yousefi-Lafouraki et al., 2014; Baffigi and Bartoli, 2010).

   1.1.3.   Nozzle Geometry

   The nozzle shape and geometry significantly influence the velocity profile, turbulence, and jet spread. Chamfered nozzles improved the heat transfer to pressure drop ratio by up to 30.8% compared to square-edged nozzles. Among orifice geometries, square nozzles outperformed circular ones, increasing cooling efficiency by 42% due to better flow penetration. Rectangular nozzles with an aspect ratio of 5 achieved the best performance, covering 14 times the nozzle area. Using grid patterns, higher thickness ratios enhanced localized heat transfer at shorter nozzle-to-plate distances by amplifying flow disturbances (Wang et al., 2022; Cafiero et al., 2017; Brignoni and Garimella 2000).

   1.1.4.   Impact Distance 

   The standoff distance, defined as the distance between the nozzle exit and the target plate, significantly affects heat transfer efficiency. Studies show that this region typically spans 4 to 6 jet diameters and influences jet intensity. For instance, at H/De = 2, chaotic flow near the surface enhances heat transfer, while at H/De = 4, increased turbulence in the stagnation zone further improves performance (He and Liu, 2018; Zou, 2001).

   1.1.5.   Resonance

   Resonance phenomena, caused by acoustic or fluid dynamic instabilities, can significantly enhance heat transfer in impinging jet systems. Oscillations or vibrations introduced into the jet increase turbulence and mixing, improving thermal performance. Resonant flow involves the formation of standing waves within a cavity, generating intense fluid motion that boosts convective heat transfer. One study found that a single fin could disrupt flow near the front edge, amplifying downstream vibrations and increasing heat convection by up to 23%. Another study experimentally investigated periodic jets, where controlling the mass flow rate improved performance. The combination of triangular (or sinusoidal) and rectangular signals yielded the best results, outperforming other configurations (Geng et al., 2015; Xu et al., 2009).

   1.2. Self-Exciting Jets

   To improve the efficiency of jet impingement in heat exchange applications, various methods can be utilized. These methods can be broadly classified into three categories such as passive, active, and hybrid. Subsequently, passive methods do not require additional energy to refine thermal exchange, while active methods incorporate supplementary devices such as fans or pumps to enhance fluid circulation and intensify thermal exchange. Passive methods do not incorporate moving components, thereby being cost-effective and more reliable than active methods. While hybrid systems involve the use of multiple passive or active systems to enhance thermal performance. (Saha et al., 2020; Pati et al., 2017; Webb and Kim 2005).

   Hybrid systems use a combination of methods to enhance heat transfer and are becoming more significant in engineering. By blending different techniques, these systems achieve better cooling and thermal management. Hybrid methods are highly valuable in industries like electronics for effective heat dissipation and in energy systems to improve thermal efficiency. For instance, combining jet impingement with multilayer microchannel heat sinks is a proven approach to manage high heat fluxes while maintaining low pressure drops. These hybrid heat sinks provide adaptable designs that can be fine-tuned to achieve specific thermal performance goals, ensuring efficient heat dissipation with minimal pressure loss. Moreover, studies show that using square multiple impinging jets with hybrid nanofluids under optimal conditions can enhance the Nusselt number by approximately 35%. Another research focused on cooling an oscillating hot square object with a double slot jet impingement system under a magnetic field. In this setup, the use of hybrid nanofluids, coupled with the object's oscillation, led to a substantial improvement in cooling efficiency. Compared to using a base fluid without a magnetic field, cooling performance increased by 62.9% at Reynolds Number (Re) = 100 and 77.6% at Re = 500 when both hybrid nanofluid and object oscillation were applied (Ajeel et al., 2024; Selimefendigil et al., 2024; Mostafa et al., 2023). Recently, the use of AI (Artificial Intelligence) has gained significant importance in predicting, optimizing, and monitoring designs with high accuracy. A numerical study using an Artificial Neural Network (ANN) assisted Computational Fluid Dynamics (CFD) method was conducted on a hybrid cooling system that combines channel flow and slot jet impingement to cool an elastic plate. The results show that this combination significantly improves the cooling performance of both the upper and lower sections of the plate, while also addressing its deflection. Another study achieved an accuracy of 99.03% for the average Nusselt number and 94.82% for the Sherwood number when analyzing the magneto-bioconvective behavior of hybrid nanofluid in an inclined porous annulus with oxytactic microorganisms using ANN and CFD. (Swamy et al., 2024; Selimefendigil and Oztop 2024)

   The flow chart presented in Figure 2 illustrates heat transfer enhancement through active and passive excited jets, with further improvement achieved by combining excitation jet methods with other methods such as fluid diode, surface modification, piezoelectric fan, nanofluids, and nozzle geometry which will be the focus of this study. The use of passive or active methods to excite turbulent jets has a significant interest in thermal engineering due to its theoretical and practical importance for enhancing jet turbulence and mixing characteristics. Numerous experimental and theoretical analyses have explored both passive and active excitation jets, examining the impact of different shapes and working conditions on fluid flow and heat exchange properties (Sabato et al., 2019; Persoons et al., 2011; Chaudhari et al., 2010a; 2010b; Tesa? et al., 2006; Lee et al., 2003).

   

   Figure 2 Flow chart of the study

   
   

   1.2.1.   Passive Self-Exciting Jets

   Passive self-excited jets leverage the inherent characteristics of jet flow to enhance heat exchange without requiring additional energy input. Among the most notable examples are annular, swirl, and sweeping jets (Khan et al., 2023; Fénot et al, 2021; Eghtesad et al., 2019; Ikhlaq et al., 2019; Park et al., 2018; Trávní?ek and Tesa?, 2013; Bakirci and Bilen 2007; Lee et al., 2002).

   1.2.1.1.  Annular Jets

   Annular jets, produced by a ring-shaped nozzle, generate a hollow cylindrical flow. This configuration enhances the surface area of the impinging fluid, thereby improving heat exchange rates. The structure of an annular jet involves the creation of a vortex region beyond the nozzle outlet, featuring two axisymmetric shear layers: one at the outer boundary of the annular flow and another on the inner surface. The interaction of the flow between these different regions leads to significant large-scale turbulent disturbances. Studies have shown that at identical mass flow rates, the annular jet displayed notably greater velocity and turbulent fluctuations compared to the circular jet (Terekhov et al., 2016; Patte-Rouland et al., 2001).

   In specific configurations of jet exit Re, critical impact distance (H), and a specific blockage ratio (BR), thermal exchange for the annular jet was enhanced by 10-15% relative to the circular jet. The optimal thermal improvement relative to the circular jet occurred at approximately BR=0.4. Additionally, the impact of extending the central shaft of the annulus past the exit point of jet, considering three different shapes including cylindric, conic, and half-ellipsoid was examined. Thermal exchange was influenced differently depending on the configuration and size of the bullet extension. Relative to the standard case, the ellipsoidal extension had a slight positive impact on average heat transfer, particularly at higher Re, with improvements of about 1- 2%, the cylindrical extension provided thermal enhancement for longer extensions (more than the outer jet diameter), while the conic extension reduced thermal exchange regardless of the extension length. As the extension length increased, there was a slight increase in maximum Nu values (Afroz and Sharif 2022; 2018).

   1.2.1.2. Swirling Jets

   Swirl jets are created by introducing a tangential component to the fluid flow, causing jet to spin around its axis. The dynamic interplay between the upstream and downstream flow regions was examined, focusing on the impact of impingement on vortex breakdown induced by swirl and the intricate relationship between thermal exchange and the downstream flow domain. In the upstream region, under minimal swirl conditions around 0.3, free and impinging jets each exhibit an absence of vortex failure. However, at a higher swirl rate of around 0.74, vortex failure is observed only in Re range between 11,600 and 24,600. At close-range interaction distances of H/D = 2 and 4, the recirculation bubble associated with vortex breakdown appears more spherical and exhibits greater size and strength at lower Re, diminishing at higher Re (Ikhlaq et al., 2021).

   The emergence and evolution of organized vortical patterns, influenced by the submerged impinging swirling jet, play a significant role in shaping flow characteristics and the conveyance of thermal energy, acting as a passive variable in the given operational parameters (Contino et al., 2022).

   The intensity of swirl diminishes as jet-wall distance increases. Conversely, the impact of swirl becomes more pronounced at a larger jet-wall distance, resulting in a wider affected area along the wall. Furthermore, although swirl motion contributes to turbulence increase, thermal coefficient declines with a higher swirl number. Swirl motion improves the uniformity of thermal exchange for both jet-wall distances, except near the stagnation region at H/D = 2, where variations in Re influence the enhancement (Huang et al., 2022).

   A numerical method showed a better swirling enhancement (at L/D = 6 and 8). At higher Re, thermal exchange rate increases because of the high rate of momentum transfer. Notably, the use of two jets with identical flow rates can raise heat rate by 10% (Amini et al., 2015).

   1.2.1.3. Sweeping Jets

   Sweeping jets, which oscillate back and forth across the target surface, can effectively cover a significantly larger area, extending to nearly half of a large curved surface, with a robust impingement effect. In addition, as Re increased and the gap between the nozzle and the wall decreased, thermal exchange of the sweeping jet experienced a boost. At a high Re of 15,000, the sweeping jet demonstrated notable improvements in the domain at H/D ratios of 0.5 and 1.0, reaching up to a remarkable 40% increase around the stagnation zone. Additionally, by excluding the outer diffuser at a distance ratio (X/D) of 4.5 and Re of 60,000, a 140% enhancement in thermal exchange was observed. Conversely, increasing the distance ratio and divergence angle leads to a decrease in thermal exchange coefficient. In the confined impingement scheme, the sweeping jet demonstrates superior overall cooling efficiency relative to the steady jet. However, in the unconfined impingement configuration, the steady jet exhibits a slight advantage (Abdelmaksoud and Wang 2023; Joulaei et al., 2022; Wen et al., 2019; Zhou et al., 2019a).

   1.2.2.   Active Exciting Jets

   Actively excited jets require external energy sources, such as fans or pumps, to enhance heat transfer. They include synthetic and pulsed jets, which offer precise control over jet flow characteristics (Ergur and Calisir 2023; Castellanos et al., 2023; Kim et al., 2022; Kurian et al., 2021; Gil and Wilk 2020; Travnicek and Antosova 2020; Raizner et al., 2019; Zhang et al., 2018; Larasati et al., 2017. Kosasih et al., 2016).

   1.2.2.1 Synthetic Jets

   Synthetic jets are generated by the cyclic ejection and suction of fluid through an orifice, typically driven by a diaphragm or piezoelectric actuator. The sinusoidal waveform exhibits a significantly higher average Nusselt number, outperforming the square waveform by 20.86%. However, in the region of no flow separation, the square waveform surpasses the sinusoidal waveform by 18.94% in thermal exchange. Notably, the sinusoidal waveform achieves a 73.68% higher mass flow rate relative to the square signal at a resonance frequency of 150 Hz. These jets operate without the need for a continuous supply of external fluid, making them both efficient and compact. Meanwhile, investigations into the effect of orifice plate thickness on thermal transport behavior revealed that the peak heat transfer coefficient was 16.1 times higher than that of natural convection (Singh et al., 2022; 2020).

   Experiments conducted using synthetic jets emerging from a star-shaped nozzle with lobes ranging from 3 to 10 revealed that the 6-lobed star orifice demonstrated optimal thermal performance at distance to target over diameter z/d ratio ranging from 2 to 14. At z/d = 2, the 6-lobed orifice outperformed the round orifice by 7.6% and the 10-lobed star orifice by 10.1%, due to the generation of vortices along the edges of the lobes, resulting in a subsequent shift of the axis between the primary and secondary planes. The square nozzle shape and sinusoidal wave frequencies provide a wider coverage area compared to the circular nozzle. This results in a higher entrainment rate, which enhances heat transfer performance (Sharma et al., 2023; Harinaldi et al., 2013).

   Increasing the pulse frequency in the synthetic impinging jet led to the development of local velocity and gas disturbances, enhancing thermal exchange and increasing stagnation of thermal conductivity. The stagnation Nusselt number increased for short orifice-to-wall distances (H/d < 4), with peak thermal conductivity observed at distances of H/d from 3 to 5. Further increases in distance resulted in inhibited thermal exchange (Lemanov et al., 2022).

   1.2.2.2   Pulsed Jets

   Pulsed jets involve the intermittent discharge of fluid at high velocities, with control over frequency and amplitude. The introduction of pulsation caused jet to spread, resulting in a reduction of the potential core length compared to a normal jet. Pulsation amplified the mixing and diffusion processes with the surrounding fluid, thereby enhancing thermal exchange in the potential cone area. Using a pulsating jet operating at a frequency range of 40 to 160 Hz resulted in a 2 to 8% increase in the time-mean Nusselt number in the wall jet zone. In addition, the impacts of square-signal pulsating frequency and amplitude were investigated, providing detailed insights into the interaction of these parameters on cooling performance (Wang et al., 2023; Marzouk et al., 2022; Zargarabadi et al., 2018).

   Introducing a pulse frequency of 100Hz to jet resulted in a notable 22% enhancement in the convective heat transfer coefficients for round/square jets and a 15% improvement for elliptical/rectangular jets. The pulsation exhibited a dual effect on the average Nusselt number, leading to a reduction at low frequencies and an increase at high frequencies compared to steady jets (Rakhsha et al., 2023).

   After introducing all types of excitation jets, the focus shifts to hybrid methods, which combine elements from both passive and active methods to optimize thermal performance and are the main focus of this review. This review study explores the potential of hybrid methods in providing superior heat transfer by integrating the advantages of various excitation methods. The subsequent sections will explore specific hybrid configurations, their influence on thermal enhancement, and how they combine the strengths of passive and active jets to optimize efficiency. Through detailed analysis and experimental results, the next section aims to demonstrate how hybrid methods can be effectively employed for advanced thermal management solutions.

Hybrid Methods for Enhancing Heat Transfer

In the search for improved heat transfer performance, hybrid methods have emerged as a compelling solution. These methods combine the strengths of different thermal enhancement strategies to achieve superior results. By integrating various passive and active methods, such as advanced fluid dynamics, innovative materials, and external control mechanisms, hybrid methods can significantly enhance the efficiency of thermal management systems. These combinations lead to increased turbulence, better mixing, and more efficient heat dissipation, making them highly effective for a wide range of applications in fields such as electronics cooling, industrial processes, and energy systems. The synergy between different approaches in hybrid methods represents a powerful method to overcoming the limitations of traditional heat transfer methods and achieving optimal thermal performance (Bai et al., 2020; Lyu et al., 2019a; 2019b; Chouaieb et al., 2017).

2.1. Heat Transfer Enhancement by Combining Excited Jets with Fluid Diode

In the pursuit of advanced heat transfer solutions, hybrid methods that integrate various enhancement strategies have gained significant attention. One such innovative method is the combination of synthetic jets with fluid diode technology. The implementation of fluid diodes is designed to regulate oscillating airflow, thereby suppressing the reversal of hot air through the nozzle exit during the suction phase. Studies have highlighted the superior cooling capabilities of hybrid synthetic jet, which reduces the average Nusselt number by 54.7% relative to a simple synthetic jet configuration, offering valuable insights guiding the development of novel hybrid synthetic jets and emphasizing their potential for high-efficiency impingement cooling applications. On the other hand, the use of pulsation with the fluid increases Re, optimizing heat and mass exchange and enhancing flow oscillations, resulting in a 12 to 40% improvement in heat and mass transfer. Additionally, hybrid synthetic jet achieved an 18% enhancement in heat transfer rate due to a 26% greater flow rate compared to the standard synthetic jet (Yu et al., 2019; Trávní?ek and Vít, 2015).

2.2. Heat Transfer Enhancement by Combining Excited Jets with Surface Modification

Using a pulsating jet with frequencies between 50 and 100 Hz towards a rough wall equipped with 12 elliptical pins significantly amplifies heat transfer rate, making it more effective compared to a round pin. The optimal placement for mounting the pin on a flat wall is at a distance equal to three times the diameter of jet (r = 3D). Another study, examining a rough surface featuring 36 elliptical pin-fins of varying diameters, reported a significant 35% improvement in thermal exchange as the orifice-to-wall distance increased under pulsating flow conditions. In contrast, under steady-state flow conditions at Re of 10,000, a reduction of approximately 12% was observed, with the effect of the impact distance being minimal at higher Re. The aspect ratio played a significant role in improving thermal exchange, showing a 20% Improvement in average Nusselt number with a rise in pin height, while a reduction in average Nusselt number was observed with a rise in pin diameter for AR > 1. Additionally, introducing oscillation at a frequency of 100 Hz, relative to the normal state, was found to boost thermal exchange by up to 17% (Yousefi-Lafouraki et al., 2023; 2022).

Another study assessed the efficiency of heat dissipation by combining the use of single and multiple synthetic jets with hollow cylindrical fins filled with PCM. The performance improved with this incorporation, and the best results were achieved when using Eicosane in the impinging wall and exposing it to multiple orifices synthetic jets. The configuration yielded thermal resistance of 0.91 K/W, thermal coefficient of 420.4 W/m², and a Coefficient of Performance (COP) of 2.1. These results provide a valuable enhancement to thermal efficiency in such systems (Arumuru et al., 2023).

2.3. Heat Transfer Enhancement by Combining Excited Jets with Piezoelectric Fan

Thermal efficiency improvement can be achieved by combining excitation jets with piezoelectric fans. The addition of a piezoelectric fan to the circular jet has an advantage on thermal performance, leading to a 20% improvement in the area-averaged Nusselt number at a small impact distance relative to the circular jet. This combined method optimizes heat transfer process for more efficient and compact cooling solutions (Zhou et al., 2019a).

The integration of a piezoelectric fan with a continuous circular jet was investigated, as illustrated in Figure 2. A similar configuration utilizes the fan's vibrations to introduce pulsating excitations to jet impingement while maintaining its continuous flow. The results indicate that in the axial arrangement, the average thermal exchange optimization factor is improved by 13%, and thermal exchange uniformity factor by 0.5%, relative to the transverse system (Li et al., 2024).

Figure 3 Configuration of a nozzle with the piezoelectric fan

Jet excitation leverages fluid flow to disrupt thermal boundary layer, while piezoelectric fans induce mechanical vibrations that enhance fluid mixing and promote heat transfer. The integration of synthetic jets with a piezoelectric fan exemplifies the effectiveness of this interaction. Combining synthetic jets with a piezoelectric fan demonstrates this effective interaction. The synthetic jet, produced by the piezoelectric actuator, achieves the peak performance when operated at its optimal frequency. Additionally, studies have explored the combination of pulsed jets with a piezoelectric micropump. By adjusting the signal applied to the piezoelectric micropump, they were able to control the pulsating jet frequency, ranging from 10 Hz to 40 Hz. These results revealed that a high-frequency pulsating jet could substantially enhance the critical heat flux, achieving a value of 245.2 W/cm² at a flow rate of 60 ml/min. This represents an 84% increase relative to a steady jet (Fan et al., 2024; Tan and Zhang, 2013).

2.4. Heat Transfer Enhancement by Combining Excited Jets with Nanofluids 

Combining pulse jets with nanofluids has shown significant potential in enhancing thermal exchange rates due to the synergistic effects of fluid pulsation and the superior thermal properties of nanofluids. Studies explored the combination of Al?O?-H?O nanofluid, with volume fractions ranging from 0% to 3.5%, and periodic pulse jets with pulsation frequencies (f) between 10 and 50 Hz, employing various waveforms (Rectangular-wave and Triangular-wave). The results indicated that the Triangular-wave conducts a better enhancement of heat conductivity with medium frequency and large Re and nanoparticles volume fraction (Li et al., 2019).

Further investigation has underscored the effectiveness of hybrid nanofluids in pulse jet systems. A study highlighted the use of Al?O?-MWCNT/water hybrid nanofluid with volume fractions ranging from 0.05-0.3% in a pulse jet with pulsating frequencies between 0.2-20 Hz, employing sine, square, and triangular waveforms. The results indicated a maximum heat transfer improvement of 24% for a 0.3% volume fraction of Al?O?-MWCNT/water relative to de-ionized water under continuous jet impingement. Moreover, a 20% improvement was observed with a sine waveform at a 0.3% volume fraction and a frequency of 0.2 Hz. Pulsating jet impingement generally produces greater cooling rates, offering valuable insights for optimizing thermal exchange through pulsating hybrid nanofluid jet impingement cooling (Atofarati et al., 2024).

2.5. Heat Transfer Enhancement by Combining Excited Jets with Nozzle Configuration

Innovative jet configurations can significantly enhance heat transfer rates by leveraging various fluid dynamics and cooling mechanisms. A study combined a water circular jet with an air annular jet in the form of a mist. When the mist concentration is significantly reduced, the primary heat transfer mechanism is identified as intermittent heat perturbations resulting from the impingement of mist droplets. As the mist concentration rises, the significance of evaporative cooling from the liquid film on the wall and convective cooling due to the flow of the film grows more pronounced in the overall heat transfer process (Quinn et al.,  2017).

In air coaxial jet configurations, enhanced mixing and flow dynamics are important for superior thermal performance. The local thermal exchange distribution displayed two peaks, one at the stagnation point and another near a unitless radial distance of 1. The increased involvement of the annular jet led to enhanced flow mixing, the extension of the dynamic regions towards the outer edges of the impingement plate, and a thinner heat gradient layer. Consequently, the use of a coaxial jet demonstrated significant advantages over a conventional single-round jet, particularly in achieving greater consistency and efficiency in thermal exchange. Alternatively, the impact of the combined swirl and circular jets reveals the complex dynamics affecting heat transfer uniformity, convective intensity, and pressure distribution in the studied impinging jet configuration. Nusselt number is enhanced by 22.2% with a rising flow ratio and by 9.3% with a lowering orifice-to-wall distance (Markal 2018; Markal and Aydin 2018).

Different nozzle shapes, such as chevron nozzles, can significantly influence synthetic jet thermal exchange performance. A study examining the combination of the synthetic jet with the pipe transmission and exit nozzle shape showed that the chevron nozzle proves to be beneficial for enhancing synthetic jet thermal exchange. Its impact varies with different orifice-to-wall distances and functional frequencies. At lower functional frequencies, the presence of the chevron exit alters the optimal orifice-to-wall distance for synthetic jet impingement thermal exchange relative to the circular exit nozzle (Lyu et al., 2019a).

Incorporating a central synthetic jet in the square-shaped array-jet assembly effectively enhances total thermal exchange efficiency, specifically when Re of the synthetic jet matches or exceeds that of the array-jet, practically making it suitable for applications. The results showed that thermal exchange enhancement persists at Re = 5000, with Nusselt numbers increasing by 30%-60% across impinging distances. Even at Re = 7000, a rise of 10%-30% was observed, and at Re = 10,000, there was an increase of less than 20% (Tan et al., 2023).

3. Evaluation of Hybrid Heat Transfer Techniques

Table 1 shows an overview of all studies discussed in hybrid section. Fluid diodes improve jet flow control, disrupting the thermal boundary layer for localized cooling and higher heat transfer rates. Piezoelectric fans amplify fluid mixing through mechanical oscillations, with optimal operation significantly boosting the heat transfer coefficient. Nanofluids enhance thermal conductivity and reduce resistance, enabling efficient energy distribution. Surface modifications, such as microstructures or conductive coatings, increase the effective heat transfer area and improve dissipation. Optimized nozzle designs further refine jet impingement patterns, ensuring uniform cooling and maximizing thermal efficiency.

Table 1 Summarize the various hybrid methods in impinging jets used to enhance heat transfer 

Table 1 Summarize the various hybrid methods in impinging jets used to enhance heat transfer (Cont.) 

Table 1 Summarize the various hybrid methods in impinging jets used to enhance heat transfer (Cont.) 

Table 1 Summarize the various hybrid methods in impinging jets used to enhance heat transfer (Cont.) 

Table 1 Summarize the various hybrid methods in impinging jets used to enhance heat transfer (Cont.) 

Table 1 Summarize the various hybrid methods in impinging jets used to enhance heat transfer (Cont.) 

Table 1 Summarize the various hybrid methods in impinging jets used to enhance heat transfer (Cont.) 

Table 1 Summarize the various hybrid methods in impinging jets used to enhance heat transfer (Cont.) 

Table 1 Summarize the various hybrid methods in impinging jets used to enhance heat transfer (Cont.) 

Table 1 Summarize the various hybrid methods in impinging jets used to enhance heat transfer (Cont.) 

In conclusion, the incorporation of a fluid diode significantly improves the directional control of the impinging jet, leading to more efficient disruption of thermal boundary layer. This leads to improved localized cooling and a significant increase in heat transfer rates. Incorporating a piezoelectric fan adds mechanical vibrations that enhance fluid mixing, further boosting heat dissipation. Operating the fan at its optimal frequency maximizes the cooling effect, significantly boosting the overall heat transfer coefficient. Utilizing nanofluids in the impinging jets improves thermal conductivity and heat capacity compared to conventional fluids. The nanoparticles in nanofluids contribute to better energy distribution and reduced thermal resistance, leading to a more efficient heat transfer process. Surface modifications, such as micro-structuring or coating with thermally conductive materials, enhance thermal properties of the surface and increase the effective surface area for heat transfer. These modifications result in improved heat dissipation and lower thermal resistance. The optimization of the nozzle configuration, including the shape and size, enhances jet impingement pattern and improves the uniformity and intensity of cooling. This leads to a more effective distribution of cooling across the target surface and maximizes heat transfer efficiency. This study shows that a multifaceted method incorporating fluid diodes, piezoelectric fans, nanofluids, surface modifications, and optimized nozzle configurations can significantly enhance heat transfer performance. Each component contributes uniquely to the overall system, leading to a synergistic effect that maximizes thermal efficiency and cooling capacity. The combination of these advanced methods offers a robust solution for high-performance thermal management in various applications. Future research should focus on exploring new hybrid combinations and understanding their dynamic interactions. Potential areas of investigation include assessing the stability and thermal performance of nanofluids under extreme conditions and prolonged use. Additionally, the development of innovative surface treatment techniques that improve durability and thermal properties can further enhance heat transfer. Leveraging computational optimization tools and artificial intelligence to create advanced nozzle geometries can also improve cooling performance and uniformity. Hybrid impinging jet techniques represent a step-change in the field of thermal engineering. By addressing challenges related to implementation, cost, and scalability, these methods have the potential to meet the growing demands for efficient and sustainable cooling solutions. Artificial intelligence can play a transformative role in this field by enabling the design of optimized jet configurations and hybrid systems through machine learning algorithms. AI can analyze large datasets from experiments and simulations to predict heat transfer performance, identify optimal operating conditions, and even automate real-time adjustments in complex thermal management systems. Interdisciplinary collaboration, incorporating AI tools, and continued innovation will be key to realizing the full potential of these approaches in future applications.

     The authors wish to thank FEDER, the 'Region of Nouvelle Aquitaine'.

Author Contributions

     Conceptualization, Michel Matar, Hassan H. Assoum, Anas Sakout, Mohammad El Hassan; methodology, Michel Matar, Hassan H. Assoum, Mohammad El Hassan; writing-original draft preparation, Michel Matar, Hassan H. Assoum, Mohammad El Hassan, Nikolay Bukharin; writing-review and editing, Michel Matar, Hassan H. Assoum, Mohammad El Hassan, Nikolay Bukharin; project administration, Hassan H. Assoum and Ali Hammoud; and funding acquisition, Anas Sakout. All authors have read and agreed to the published version of the manuscript.

Conflict of Interest

The authors declare no conflict of Interest.

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