Experimental Study of Heat Transfer and Pressure Drop Over an Array of Short Micro Pin Fins

Studies on thermal enhancement for electronic chips has been gaining prominence as increased transistor density in the chips calls for larger heat dissipation. Various enhancement techniques have been proposed ever since 1981, to enhance the heat dissipation from the chip surface. Micro pin fins have been gaining recognition as a highly favorable surface enhancement due to the design versatility it provides in the form of myriad geometric shapes and fin arrangements as opposed to convention microchannels. The micro pin fins however, present a larger pressure drop over the surface as compared to other conventional methods which reduces the thermal efficiency of the chip surface. To reduce the pressure drop associated with micro pin fins, short micro pin fins were proposed. A short micro pin fin arrangement is similar to micro pin fin arrays, with one change, in that short micro pin fins have a clearance between the fins and the top of the channel. The current study focusses on heat transfer and pressure drop over short micro pin fin arrays. Experimental studies were conducted over 10 mm × 10 mm with fin heights varying from 200 to 500 µm and clearance over the fins varying from 265 to 900 µm. Distilled water was used as the cooling medium. The heat transfer coefficient and pressure drop characteristics were evaluated at varying fin heights and varying clearance of the surfaces with an aim to identify optimum fin height and clearance parameters. The heat transfer coefficient and pressure drop data obtained from experiments were also evaluated with the correlation proposed by Tullius et al. [17]. Data showed that the highest heat transfer coefficient was observed for fins with the largest fin height. When fin clearance was evaluated for its effect on heat transfer coefficient, a hint of mixing phenomenon leading to enhancement in heat transfer coefficient was observed at higher clearance values. A higher pressure drop was observed at longer fins owing to the increased friction factor at the fin walls. The highest pressure drop of over 100 kPa was observed for a chip gasket combination which consisted of the longest fins with the least amount of clearance. It was also observed that the Nusselt number and Pressure drop correlations proposed by Tullius et al was not able to accurately predict the experimental data. However, the correlation did show the same trend as the experimental data, hence, the present correlation could be modified or used as a basis for new correlations of Nusselt number and friction factor

Micro-PIV visualization and numerical simulation of flow and heat transfer in three micro pin-fin heat sinks

This paper presents the experimental results of laminar flow behavior of water in circular micro pin-fin (C-MPF), square micro pin-fin (S-MPF) and diamond micro pin-fin (D-MPF) heat sinks using micro-PIV flow visualization technology at first. All three micro pin-fin heat sinks have a hydraulic diameter of 200 μm. Second, numerical simulation results of the fluid flow characteristics in these heat sinks with CFD are compared to the experimental results of fluid flow behaviors measured with the micro-PIV flow visualization. The normalized time averaged streamline patterns and instantaneous velocity contours in the three heat sinks were obtained for laminar flow of Reynolds number from 10 to 200. By comparison, the experimental results favorably agree with the simulated results of fluid flow. Of the three types of heat sinks, the vortexes occur the earliest in the D-MPF heat sink, which also present very complicated back flow. The strong vortexes and back flow effectively enhance the mixing of fluid and therefore lead to higher pressure drops in the D-MPF heat sink as compared to the other two types of heat sinks. The vortexes in the D-MPF heat sink are very much easily involved in the main flow than those in the other two types of heat sinks due to the high deceleration and pressurization zone. Finally, numerical simulation results of heat transfer at steady state in the three heat sinks are presented. The initial temperature of the working fluid and the ambient air is maintained at 293 K and a constant heat flux of qw = 400 kW/m2 is adopted in the central area at the bottom of the heat sink. The Reynolds number ranges from 40 to 300 for the fluid flow and heat transfer simulations. It shows that D-MPF heat sink has better heat transfer performance than the other two type heat sinks. The combined effects of the vortex in the main flow at the front side wall and the strong vortex intensity behind the D-MPF heat sink obtained in both experimental and numerical results may reasonably explain the better heat transfer enhancement behaviors as compared to those in the other two types of heat sinks. Further experiments on the heat transfer performance will be conducted to compare to the simulated results in the follow-up planned research

Transport Characteristics of Pin Fin Enhanced Microgaps under Single and Two Phase Cooling

Microfluidic convection cooling is a promising technique for future high power microprocessors, radio-frequency (RF) transceivers, solid-state lasers, and light emitting diodes (LED). Three-dimensional (3D) stacking of chips is a configuration that allows many performance benefits. A microgap with circulating fluid is a promising cooling arrangement that can be incorporated within a 3D chip stack. Although studies have examined the thermal characteristics of microgaps under both single-phase and two-phase convection, the characteristics and benefits of microgaps with surface enhancement features have not been fully explored. In this work, firstly, the single phase thermal/fluid characteristics of microgaps with staggered pin fin arrays are studied. The effects of the pin fin dimensions including diameter, transversal and longitudinal spacing, and height are investigated computationally and experimentally over a range of Reynolds number (Re) 22-357. Micropin fin arrays investigated have pin diameter of 100 μm, pitch/ diameter ratios of 1.5 ~ 2.25, and height/ diameter ratios of 1.5 ~ 2.25. Correlations of friction factor (f) and Colburn j factor for these dense arrays of micro pins have been developed. Subsequently, microfluidic cooling with staggered pin fin arrays is employed in functional 3D integrated circuit (ICs). Thermal and electrical performance of a CMOS chip in terms of temperature and leakage power under realistic operating conditions are studied. Both experimental and modeling results show that microfluidic cooling could significantly decrease the chip temperature and leakage power, thus increasing the chip performance. Lastly, two-phase cooling is studied with dielectric fluid HFE-7200 as a baseline with mass flux from 354.5 kg/m2-s to 576.3 kg/m2-s. Critical heat flux (CHF) increases with increasing mass flux but decreases with decreasing gap height. Nonuniform heating will cause nonuniform flow with a decrease of mass flux in high power area, which decreases the thermal performance. The effects of fluid mixture (HFE-7200/Methanol) on thermal performance are studied with mass fraction of Methanol from 8.5% to 35.8%. A very small amount of addition of Methanol (8.5% mass fraction) can significantly increase the thermal performance due to the sharp decrease of saturation temperature and increase of effective thermal conductivity and latent heat. However, the Marangoni effect caused by the concentration gradient deteriorates the CHF.Ph.D

MICRO HEAT EXCHANGER TO COOL CEREBROSPINAL FLUID FOR BRAIN INJURY TREATMENT

Hypothermia is accepted as a method to preserve cells and tissue. Clinical evidence shows that administration of hypothermia could lead to neuroprotection after cardiac arrest. Non-invasive methods such as surface cooling devices, drugs and cold liquid ventilation are available to induce hypothermia. These approaches for achieving hypothermia have not been optimized yet. The surface cooling methods are generally slow and may lead to additional thermal shock to the body. Here, we propose a rapid, selective cooling method for the brain using a micro heat exchanger to cool the cerebrospinal fluids (CSF). We designed and 3D printed a U-type heat exchanger utilizing UV resin as the material for the heat exchanger as it has good mechanical and thermal properties. The heat exchanger has the inlet and outlet at the centre on both ends. There are seven channels for fluid flow and eight fins around the channels. The heat exchanger is placed on a Peltier component to keep the temperature constant across the heat exchanger. A syringe filled with CSF was utilized, which was placed on a syringe pump to provide the fluid at the inlet through the tube connections. To test the heat exchanger\u27s efficiency, an artificial CSF was allowed to flow through it, and the surface and the outlet temperature were captured. Various parameters were optimized, such as the flow rate, the initial CSF temperature at the inlet, the voltage to be supplied to run the Peltier. This report presents theoretical and experimental results for Micro Heat Exchanger. Fluid flow behavior was investigated analytically as well as using a CFD code (ANSYS Fluent). The theoretical results were validated with the experimental results by measuring the surface and fluid temperature of the heat exchanger at specific locations. Overall, we saw a close agreement between the simulated and experimental results for the surface and outlet temperature. The U-type heat exchanger micromodel will improve the understanding of complex flow patterns in 3D and open a new approach for treating brain injuries in humans and animals with small form factors

Experimental study on heat transfer from rectangular fins in combined convection

Combined natural and forced convective heat transfer arise in many transport processes in engineering devices and in nature, which is frequently encountered in industrial and technical processes, including electronic devices cooled by fans, heat exchangers placed in a low-velocity environment, and solar receivers exposed to winds. In this study, the effects of design parameters have been experimentally investigated for the air-side thermal performance under combined (natural and forced)convection of the rectangular plate heat sinks, and the values of optimum design parameters were sought. Many ideas for improving cooling methods have been proposed, one of which is the heat sink. In this work, the average Nusselt number (Nu) and thermal resistance of a simple base rectangular plate and five vertical rectangular plate heat sinks with different numbers of fins under natural and combined convection were experimentally investigated to obtain the maximum average Nu and minimum thermal resistance for various Reynolds numbers (Re) from 2300 to 40000, Rayleigh numbers (Ra) from 1300000 to 13000000, and Richardson numbers (Ri) from 0.4 to 3. Also, in this experiment, fin spacing (P) was varied from 2.8 mm to 14.6 mm and the dimensionless P/H ratio was varied from 0.1 to 0.49. The flow velocity varied in the range of 2 to 8 m/s under combined convection. Based on the effects of Ri and Re, two empirical equations for natural and also for combined convection heat transfer were derived to calculate the average Nu. The average deviation for these two equations is about 7%.The outcomes of this research can be beneficial for engineers who work on electronics cooling systems

Improving Small Scale Cooling of Mini-Channels using Added Surface Defects

Advancements in electronic performance lead to a decrease in device size and an increase in power density. Because of these changes, current cooling mechanisms for electronic devices are beginning to be ineffective. Microchannels, with their large heat transfer surface area to volume ratio, cooled with either gas or liquid coolant, have shown some potential in adequately maintaining a safe surface temperature. By modifying the walls of the microchannel with fins, the cooling performance can be improved. Using computational fluid dynamics software, microfins placed in a staggered array on the bottom surface of a rectangular minichannel are modeled in order to optimize microstructure geometry and maximize heat transfer dissipation through convection from a heated surface. Fin geometry, dimensions, spacing, height, and material are analyzed. Correlations describing the Nusselt number and the Darcy friction factor are obtained and compared to recent studies. These correlations only apply to short fins in the laminar regime. Triangular fins with larger fin height, smaller fin width, and spacing double the fin width maximizes the number of fins in each row and yields better thermal performance. Once the effects of microfins were found, an experiment with multi-walled carbon nanotubes (MWNTs) grown on the surface were tested using both water and Al2O3/H2O nanofluid as the working medium. Minichannel devices containing two different MWNT structures – one fully coated surface of MWNTs and the other with a circular staggered fin array of MWNTs - were tested and compared to a minichannel device with no MWNTs. It was observed that the sedimentation of Al2O3 nanoparticles on a channel surface with no MWNTs increases the surface roughness and the thermal performance. Finally, using the lattice Boltzmann method, a two dimensional channel with suspended particles is modeled in order to get an accurate characterization of the fluid/particle motion in nanofluid. Using the analysis based on an ideal fin, approximate results for nanofluids with increase surface roughness was obtained. Microchannels have proven to be effective cooling systems and understanding how to achieve the maximum performance is vital for the innovation of electronics. Implementation of these modified channel devices can allow for longer lasting electronic systems

CROSS FLOW BOILING IN MICRO-PIN FIN HEAT SINKS

ABSTRACT Flow boiling of water across a bank of circular staggered micro pin fins, 250 µm long and 100 µm diameter with pitch-todiameter ratio of 1.5, was experimentally studied for mass fluxes ranging from 346 kg/m 2 s to 794 kg/m 2 s and surface heat fluxes ranging from 20 W/cm 2 to 350 W/cm 2 . The local twophase heat transfer coefficients were measured using thermistors located along the flow path of the channel. The flow was visualized and classified as vapor slug and annular flow patterns. Based on the observed flow patterns, the dominant heat transfer mechanism during boiling process was assumed to be convective boiling. INTRODUCTION Single-phase [1-5] and two-phase [6-9] flows in microchannels have been a topic of extensive studies over the last several years. Recently, pin fin microchannel heat sinks have also been studied, but primarily in the context of singlephase flo

The design of mini/micro heat exchangers: A world of opportunities and constraints

Micro heat exchangers and heat sinks broadened their use in many technological fields during the last two decades. The reduction of the dimensions of the channels allows to obtain ultra-compact heat exchangers characterized by higher surface-to-volume ratio and overall heat transfer coefficients but, in general, with large pressure losses. Many imaginative configurations have been proposed and tested, by changing the geometry of the manifolds, the position of the inlet/outlet ports, the structure of the heat transfer core, the structural materials and others more. Unfortunately, these efforts were not coordinated and a complete overview of the results accumulated up to now is not available. However, some general conclusions can be made by using the published results and the main scope of this paper is to summarize these milestones. Some shared conclusion are the following: (i) the design of micro heat exchangers can be obtained by using the classical methods developed for conventional heat exchangers even if the presence of non- negligible scaling effects (i.e. compressibility effects, conjugate wall-fluid effects, viscous dissipation) must be always verified; (ii) the performances of micro heat exchangers and heat sinks is strongly influenced by the proper distribution of the flow rate within the heat transfer core and a series of different solutions is available in order to solve this problem, as summarized in this paper; (iii) the presence of strong conjugate wall-fluid heat transfer effects can become an opportunity for the use of miniaturized heat exchangers made with inexpensive materials having low thermal conductivity values, especially in presence of counter-current flow and cross-flow configurations