Numerical Study Of Heat Transfer From Pin Fin Heat Sink Using Steady And Pulsated Impinging Jets
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The work reported in this thesis is an attempt to enhance heat transfer in electronic devices with the use of impinging air jets on pin-ﬁnned heat sinks. The cooling per-formance of electronic devices has attracted increased attention owing to the demand of compact size, higher power densities and demands on system performance and re-liability. Although the technology of cooling has greatly advanced, the main cause of malfunction of the electronic devices remains overheating. The problem arises due to restriction of space and also due to high heat dissipation rates, which have increased from a fraction of a W/cm2to 100s of W /cm2. Although several researchers have at-tempted to address this at the design stage, unfortunately the speed of invention of cooling mechanism has not kept pace with the ever-increasing requirement of heat re- moval from electronic chips. As a result, efﬁcient cooling of electronic chip remains a challenge in thermal engineering. Heat transfer can be enhanced by several ways like air cooling, liquid cooling, phase change cooling etc. However, in certain applications due to limitations on cost and weight, eg. air borne application, air cooling is imperative. The heat transfer can be increased by two ways. First, increasing the heat transfer coefﬁcient (forced convec- tion), and second, increasing the surface area of heat transfer (ﬁnned heat sinks). From previous literature it was established that for a given volumetric air ﬂow rate, jet im-pingement is the best option for enhancing heat transfer coefﬁcient and for a given volume of heat sink material pin-ﬁnned heat sinks are the best option because of their high surface area to volume ratio. There are certain applications where very high jet velocities cannot be used because of limitations of noise and presence of delicate components. This process can further be improved by pulsating the jet. A steady jet often stabilizes the boundary layer on the surface to be cooled. Enhancement in the convective heat transfer can be achieved if the boundary layer is broken. Disruptions in the boundary layer can be caused by pulsating the impinging jet, i.e., making the jet unsteady. Besides, the pulsations lead to chaotic mixing, i.e., the ﬂuid particles no more follow well deﬁned streamlines but move unpredictably through the stagnation region. Thus the ﬂow mimics turbulence at low Reynolds number. The pulsation should be done in such a way that the boundary layer can be disturbed periodically and yet adequate coolant is made available. So, that there is not much variation in temperature during one pulse cycle. From previous literature it was found that square waveform is most effective in enhancing heat transfer. In the present study the combined effect of pin-ﬁnned heat sink and impinging slot jet, both steady and unsteady, has been investigated for both laminar and turbulent ﬂows. The effect of ﬁn height and height of impingement has been studied. The jets have been pulsated in square waveform to study the effect of frequency and duty cycle. This thesis attempts to increase our understanding of the slot jet impingement on pin-ﬁnned heat sinks through numerical investigations. A systematic study is carried out using the ﬁnite-volume code FLUENT (Version 6.2) to solve the thermal and ﬂow ﬁelds. The standard k-ε model for turbulence equations and two layer zonal model in wall function are used in the problem Pressure-velocity coupling is handled using the SIMPLE algorithm with a staggered grid. The parameters that affect the heat transfer coefﬁcient are: height of the ﬁns, total height of impingement, jet exit Reynolds number, frequency of the jet and duty cycle (percentage time the jet is ﬂowing during one complete cycle of the pulse). From the studies carried out it was found that: a) beyond a certain height of the ﬁn the rate of enhancement of heat transfer becomes very low with further increase in height, b) the heat transfer enhancement is much more sensitive to any changes at low Reynolds number than compared to high Reynolds number, c) for a given total height of impingement the use of ﬁns and pulsated jet, increases the effective heat transfer coefﬁcient by almost 200% for the same average Reynolds number, d) for all the cases it was observed that the optimum frequency of impingement is around 50 − 100 Hz and optimum duty cycle around 25-33.33%, e) in the case of turbulent jets the enhancement in heat transfer due to pulsations is very less compared to the enhancement in case of laminar jets.