|Edy Susanto||Department of Mechanical Engineering, Faculty of Engineering, Universitas Medan Area, Medan 20112, Indonesia|
|M. Idrus Alhamid||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Nasruddin||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Budihardjo||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Prabowo||Department of Mechanical Engineering, Sepuluh Nopember Institute of Technology, Surabaya 60117, Indonesia|
|Sentot Novianto||Department of Mechanical Engineering, Faculty of Engineering, Universitas Trisakti, Jakarta 11440, Indonesia|
Many efforts have been made to reduce the energy consumption of household refrigerators. One method is to place the pipe condenser to increase the rate of heat release. This numerical study examines the effect of changing the gap ratio on the flow characteristics and natural convection heat transfer of 20 pipes attached vertically to the convection plate with aluminum foil coating. The gap ratio was varied between 1.05 and 4.20 using ANSYS-FLUENT software to obtain velocity vector, temperature contour, tangential velocity, and local Nusselt number, both inside and outside the channel. With the change in gap ratios from 1.05 to 2.10, the rate of heat transfer increased significantly, reaching 2.2% while tangential velocity also increased considerably. At gap ratios of 3.15 and 4.20, the rate of heat transfer increased more gradually, local Nusselt number increased slightly where the influence of convection walls was smaller, and tangential velocity showed a very small increase. Flow characteristics were similar, with air flowing upward across the inner and outer channels.
Gap ratio; Household refrigerator; Natural Convection; Nusselt Number; Vertical Channel
Household refrigerators are an important part of daily life, and researchers have made many attempts to make them more energy-efficient. Several factors contribute to the energy-efficiency of household refrigerators, including consumer behavior and heat dissipation. One factor in consumer behavior that affects refrigerator energy consumption has been demonstrated by Susanto et al. (2018), who tested the relationship between thermostat regulation and increased energy consumption of household refrigerators. The relationship between refrigerator energy consumption and heat transfer factors was investigated by Sefcik et al. (1991) conducted experiments on natural convection behavior in enclosures that were ventilated at the lower and upper ends to provide air flow to improve heat transfer. In his experiment, the average Nusselt number changed according to the size of the two ventilation holes.
Research conducted by Davies et al. (2000) found that changes in free air temperature affect the Nusselt number: the higher the free air temperature the lower the Nusselt number. Chouikh et al. (1998) conducted a numerical study for natural convection flow along a range of two isothermal horizontal pipes by varying the Rayleigh number (Ra) and pipe spacing (Ra was used to determine the laminar to turbulent transition from the flow of natural convection boundary layers). The results showed that Ra increased and the air temperature gradient in the pipes was steeper so that the rate of heat transfer increased, and vice versa. Manca et al. (2002) studied natural convection heat-transfer characteristics using of discrete heated plates parallel slope variations; they tested the hypothesis that at an angle smaller than 85o air flow would move outside thereby increasing the temperature inside the channel. Manca et al. (2002) studied the characteristics of natural convection heat transfer using; it is stated that at a slope angle <85o it causes an inflow of air from the upper side which prevents the outflow of air thereby increasing the temperature in the channel. Buonomo et al. (2017) studied two horizontal parallel walls by filling the stem with porous media, finding that the use of porous media resulted in an increase in heat transfer. Dehghandokht et al. (2011) performed numerical analysis on a multiport serpentine meso-channel heat exchanger. Their simulation showed that the effect of serpentine bends on heat exchangers will increase average heat transfer by almost 20% compared with those using straight plates.
Lewandowski et al. (2018a) observed natural convection by varying channel width and wall temperature, the results finding that wider air ducts caused significantly heat transfer. Ospir et al. (2012) investigated dynamic flow in vertical plate channels with Rayleigh numbers and gap ratio modification; they used laser tomography to visualize flow, finding that a larger gap ratio causes the length of the upper cell to decrease. Alzwayi et al. (2014) performed numerical simulations to investigate the effect of channel width on transition flow under various plate temperatures, using the k-tur turbulence model to simulate flow and thermal fields in the channel; their results showed that the transition flow in isothermal cases is slower than adiabatic cases.
Lewandowski et al. (2018b) investigated the distribution and loss of heat from building walls using a Thermal Imaging Camera (TIC). they found that the use of infrared cameras made it possible to determine local heat loss. Lewandowski et al. (2017) also analyzed convection heat transfer to two parallel and vertical plates where the slit plate-width varies; infrared observation showed there was a relationship between the width of the gap and the rate of heat transfer.
The above studies generally conclude that the heat transfer observed occurs through natural convection. In natural convection, fluid flows due to differences in density caused by differences in temperature (buoyancy) and the absence of external influences such as fans. Serpentine pipe bends cause an increase in heat transfer. The novelty of this research is the combination of serpentine pipe bends that are attached to vertical plates, providing space for air flow. This is of interest because it causes a greater increase in heat transfer. The purpose of this research is to study the numerical effect of changing the gap ratio on the flow characteristics and natural convection heat transfer from 20 pipes that are mounted vertically to the convection plate with a layer of aluminum foil.
The numerical results obtained by varying the gap ratio from 1.05 to 4.20 led to the following conclusions: (1) The increase in the gap ratio from 1.05 to 2.10 caused a significant increase in the total condenser heat transfer (an increase of 2.2% or 6.33 Watt); (2) Because the pipe was attached to the convection plate, the air velocity, tangential velocity, and Nusselt number are increased. This causes the upstream and downstream areas to be vortex-dominated, which has a positive impact on heat transfer characteristics; (3) The air flow characteristics inside the canal tend to be the same, where free air moves from the bottom upward in the canal and convection wall due to buoyancy; (4) On the side of the pipe that adheres to the convection plate, tangential velocity and Nusselt number are zero because there is no air flowing on the side of the pipe.
The author would like to thank the Doctor Dissertation Research Grant (PDD) from the Indonesia Ministry of Research and Higher Education for funding this research (contract no. 088/SP2H/LT/DRPM/2018).
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