Thermo/fluid performance of a shielded heat sink
Highlights
► This research paper deals with the topic of electronic cooling by using plate-fin heat sink cooled by air. ► In this study we have used both numerical and experimental techniques. ► The study discusses the effect of installing thermal shield on the thermal and fluid performance of a plate-fin heat sink. ► We tried to optimize the design of the heat sink to get the optimum thermal performance with minimum pumping power.
Introduction
Over the past decade, the heat flux in electronic packages has increased drastically due to the reduction in package size combined with higher power dissipation. The efficiency of electronic products suffers from the lack of an adequate heat dissipation mechanism, possibly causing damage as the temperature rises. Therefore, effective thermal management of electronic products is of priority concern. Forced air-cooling is a popular electronic cooling approach. It is adopted widely because simplicity, economical, safe, and high reliability.
Recently, several studies on heat sink design for improved cooling of electronic component and its performance have been conducted. Morega et al. [1] investigated the minimization of the thermal resistance between a stack of parallel plates and a free stream. The best heat transfer was obtained when the plates were uniformly spaced; there existed an optimal number of plates that minimized the thermal resistance for a specified free stream and overall dimensions of the stack.
The effect of flow bypass on the performance of longitudinal fin arrays has been reported by several workers. Sparrow et al. [2] and Sparrow and Kadle [3] investigated the effect of tip clearance on thermal performance of longitudinal plate-fin heat sink. It is reported that the ratio of heat transfer coefficient with and without clearance, to be strongly affected by the tip clearance to fin height ratio, and independent of air flow rate and fin height. Writz et al. [4] studied the effects of flow bypass on the performance of ducted longitudinal heat sinks. It is reported that the value of flow bypass was found up to 60% and resulted in a reduction of the overall heat transfer rate. An optimal design of fin arrays for a given flow conditions and shroud configuration was also reported. Azar and Tavassoli [5] studied the effect of heat sink dimensions and fin density on its thermal performance. It is reported that the selection of heat sink depends not only on its thermal resistance, but also on the number of fins it has and how it is coupled to the board. Simons [6] estimated the air flow that actually passes through the fin passages of a ducted heat sink in the presence of flow bypass and also investigated the effect of this flow bypass on its thermal performance. It is reported that the flow bypass has a substantial effect on thermal performance of heat sink, and this effect can significantly increase its thermal resistance specially, with high fin density.
Elshafei [7] assessed the thermal fluid performance of a plate-fin heat sink under cross flow conditions, both experimentally and theoretically for different stream velocity and fin density. The data showed that the pressure drop increases with increasing Reynolds number and fin height but decreases with increasing inter-fin space and fin width. Jonsson and Palm [8] studied experimentally the thermal performance of plate-fin heat sink; they assessed the effects of fin height and bypass conditions on the thermal performance of various heat sinks. The thermal resistance has been found to increase with increasing height and width of the wind tunnel duct.
Thermal characteristics of plate-fin heat sink under forced air cooling were experimentally and numerically investigated using CFD code by Adam and Izundu [9]. It is reported that much of the augmentation in the heat transfer rates from heat sink is related to the increased surface area. Yuan [10] used CFD software FLUENT to study the bypass effect of the cross flow in the fin-to-fin passage of a plate-fin heat sink. The results showed that the air flow enters the fin-to-fin passage and decelerates with distance downstream, thus influencing the effect of heat transfer. Prstic and Bar-Cohen [11] compared between the thermal performances of a heat sink if it is fully shrouded, partially shrouded, or if a shield is used in the case of partially shrouded. The heat shield is placed in front of a plate-fin heat sink. It was found that the use of a thin sheet metal heat shield can decrease the bypass effect and induce additional fluid to enter the fin-to-fin channel of a plate-fin heat sink. The thermal performance of this heat shield configuration is near that of a fully shrouded configuration, but its pressure drop is markedly decreased. Li et al. [12] investigated numerically the hydraulic and thermal performance of a plate-fin heat sink undergoing cross flow forced convection with the shield. They concluded that the shield that tends to decrease the bypass flow effect has a great influence on the variation of the thermal-fluid field and enhancing the performance of the heat sink. Recently, Go-Long Tsai et.al. and [13] investigated numerically the effect of the angle of inclination of a plate heat shield on the thermal and hydraulic performance of a plate-fin heat sink. They found that the variation of this angle caused a substantial and complicated variation of the flow field in space both upstream and downstream near the heat sink. Also, Hung-Yi Li and Ming-Hung Chiang [14] investigated the effects of a shield on the thermal and hydraulic characteristics of plate-fin vapor chamber heat sinks under cross flow cooling.
The review shows that the performance of the heat sink in a cross flow is influenced markedly by the flow velocity and the geometries of the heat sink. So, this work aims at investigating experimentally and numerically the effectiveness of using a heat shield to decrease the bypass effect on thermal and hydraulic performance of a plate-fin heat sink. Moreover, the effects of changing the Reynolds number, fin number, fin height, and inclination angle of a shield on thermal performance have been investigated.
Section snippets
Experimental setup
A schematic drawing of the experimental setup used is shown in Fig. 1. The test rig is composed basically from a wind tunnel operated in suction mode as indicated by the arrows so it's called open-looped suction-type. A Plexiglas test section of same cross section as the tunnel is mounted in the middle of the wind tunnel where the heat sink and the shield are mounted. To show how the total test rig is operated, consider the inlet of the main duct as a starting point, the flow first passes
Problem description and boundary conditions
The geometry of the theoretical model and the boundary conditions are selected to fit the experimental test section. This is shown in Fig. 3. Because of symmetry, the computations are carried out only on the half of the volume (3000 mm × 125 mm × 62.5 mm) to save computational time. The thermal and flow fields were calculated numerically with commercial CFD software FLUENT 6.3.26 according to the following assumptions: the flow is steady, incompressible, and turbulent; the fluid and the solid
Shield effect on velocity and temperature fields
Inserting heat shield in the bypass region is expected to make a drastic change in velocity and temperature fields around the heat sink. So, to get insight understanding of its effect on thermal performance, the flow and temperature fields will be illustrated first for a selected design of a heat sink shown in Table 3. Some indicators are used to evaluate the performance of the heat sink. These indicators are; The Pressure Drop across the heat sink, ΔPhs, the Thermal Resistance, Rth, and the
Conclusions
The thermal performance of plate-fin heat sinks undergoing cross-flow cooling has been studied experimentally and numerically both with and without a heat shield. Based on the results of this study and the analysis of the impact of the controlling parameters, the following conclusions are drawn.
- 1.
Thermal resistance decreases significantly with fin number, fin height, and with Reynolds number. Without shield, the results showed that the thermal resistance reaches a minimum value at certain
Nomenclature
Alphabet-upper case
- At
- total convection area of a heat sink, m2
- B
- heat sink foot print width, m
- Dhd
- duct hydraulic diameter, m
- E
- total energy per unit mass, J/kg
- H
- fin height, m
- L
- heat sink foot print length, m
- N
- number of fins
- NuDh
- Nusselt number using Dh
- P1
- channel inlet pressure, N/m2
- Q
- heat input, W
- Rth
- total thermal resistance, K/W
- ReDh
- Reynolds number using Dh
- Tb
- base plate temperature, K
- Vd
- duct velocity, m/s
Alphabet-lower case
- h
- heat transfer coefficient, W/m2 K
- S
- fin spacing, m
- T
- fin thickness, m
- tb
- base plate thickness, m
- y+
- Dimensionless wall distance.
Greek symbols
- Ε
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