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

Evaluation of additively manufactured microchannel heat sinks

Microchannel heat sinks allow removal of dense heat loads from high-power electronic devices at modest chip temperature rises. Such heat sinks are produced primarily using conventional subtractive machining techniques or anisotropic chemical etching, which restricts the geometric features that can be produced. Owing to their layer-by-layer and direct-write approaches, additive manufacturing (AM) technologies enable more design-driven construction flexibility and offer improved geometric freedom. Various AM processes and materials are available, but their capability to produce features desirable for microchannel heat sinks has received limited assessment. Following a survey of commercially mature AM techniques, direct metal laser sintering (DMLS) was used in this work to produce both straight and manifold microchannel designs with hydraulic diameters of 500 μm in an aluminum alloy (AlSi10Mg). Thermal and hydraulic performance were characterized over a range of mass fluxes from 500 kg/m2s to 2000 kg/m2s using water as the working fluid. The straight microchannel design allows these experimental results to be directly compared against widely accepted correlations from the literature. The manifold design demonstrates a more complex geometry that offers a reduced pressure drop. A comparison of the measured and predicted performance confirms that the nominal geometry is reproduced accurately enough to predict pressure drop based on conventional hydrodynamic theory, albeit with roughness-induced early transition to turbulence; however, the material properties are not known with sufficient accuracy to allow for a priori thermal design. New design guidelines are needed to exploit the benefits of additive manufacturing while avoiding undesired or unanticipated performance impacts

Hydrodynamic and Thermal Flows of Fluids

The physical properties of materials  for the fluid domains, properties including density, viscosity, thermal conductivity and specific heat capacity are required for the calculation purposes. The physical properties can be assumed as dependent or independent of temperature. When there is a large temperature difference between the fluid and the surface the assumption of constant thermo-physical fluid properties may cause some errors, because in reality the thermo-physical properties of the most of the fluids vary with temperature. It is also important to note that the Prandtl number of liquids also varies with temperature, similar to that of viscosity. These property variations, of course, will affect the velocity and the temperature profile of fluid in the tube. the thermo-physical properties of working fluids are assumed as temperature dependent throughout this paper. There is working fluids used in this review paper. Keywords: therm , phys , Hagen, wall 

Numerical Investigation and Optimization of Pin Fins in Micro Heat Sinks

Operating temperature in electronics applications is continuously increasing. Therefore, for the past few decades, high heat flux removing micro heat sinks are investigated in terms of heat transfer effectiveness. This study generally concentrates on improving the passive heat transfer techniques. Micro heat sinks used in experiments are fabricated using MEMS techniques. Resistance temperature detectors, RTDs, were used for temperature measurements. The experimental data was obtained for single and two phase flow regions; however, only single phase flow results were considered in numerical simulations. Numerical validations were performed on the micro heat sinks, including cylinder and hydrofoil shaped pin fins. Following the validation phase, optimization has been performed to further improve the hydraulic and thermal performance. DOE study showed that the chord length and leading edge size of the hydrofoil pin fin are significant contributors to the thermal performance. The ranges of geometrical variables were identified and fed into multi-objective optimization cycles implementing the multi-objective genetic algorithm. The optimization objectives were to minimize pumping requirements while enhancing the local and global heat transfer effectiveness over the surface of the heater in single phase flow environment. A broad range of geometries were obtained with an acceptable tradeoff between thermal and hydraulic performance for low Reynolds number. Additionally, tandem geometries were investigated and showed that higher heat transfer effectiveness could be obtained with acceptable pumping power requirements. The importance of such optimization studies before the experimental testing is highlighted, and novel geometries are presented for further experimental investigations. Thermal performance improvement of 28% was obtained via implementing proposed geometries with only a 12% pressure drop increase. Local heat transfer optimization, aiming to decrease the local temperatures were also performed using doublet pin fin configurations. Results showed that tandem hydrofoils could control the flow with minimum pressure drops while reaching the desired low local temperatures

Heat and fluid flow in microscale from micro and nano structured surfaces

The use of enhanced surfaces became one of the most popular studies in order to increase heat transfer performances of microsystems. There are various techniques/processes applied to surfaces to enhance excess heat removal from microsystems. In parallel to these research efforts, various micro and nano structured surfaces were evaluated in channel flow, jet impingement and pool boiling applications. In the first study, single micro pin-fins having the same chord thickness/diameter but different shapes are numerically modeled to assess their heat transfer and hydraulic performances for Reynolds number values changing between 20 and 140. The pin-fins are three dimensionally modeled based on a one-to-one scale and their heat transfer performances are evaluated using commercially available software COMSOL Multiphysics 3.5a. Navier-Stokes equations along with continuity and energy equations are solved under steady state conditions for weakly compressible and single-phase water flows. To increase the computational efficiency, half of the domain consisting of a micro pin-fin located inside a micro channel, is modeled using a symmetry plane. To validate the model, experimental data available in the literature are compared to simulation results obtained from the model of the same geometrical configuration as the experimental one. Accordingly, the numerical and experimental results show a good agreement. Furthermore, performance evaluation study is performed using 3D numerical models in the light of flow morphologies around micro pin-fins of various shapes. According to the results obtained from this study, the rectangular-shaped micro pin fin configuration has the highest Nusselt number and friction factor over the whole Reynolds number range. However, the cone-shaped micro pin-fin configuration has the best thermal performance index indicating that it could be more preferable to use micro pin fins of non conventional shapes in micro pin fin heat sinks. In the second study, the results of a series of heat transfer experiments conducted on a compact electronics cooling device based on single and two phase jet impingement technique are reported. Deionized and degassed water is propelled into four microchannels of inner diameter 500 μm, which are used as nozzles and located at a nozzle to surface distance of 1.5mm. The generated jet impingement is targeted through these channels towards the surface of two nanostructured plates with different surface morphologies placed inside a liquid pool filled with deionized-water. The size of these nanostructured plates is 35mm x 30mm and they are composed of copper nanorods grown on top of a silicon wafer substrate of thickness 350 μm coated with a 50 nm thick copper thin film layer (i.e. Cu-nanorod/Cu-film/Silicon-wafer). Nanorods were grown using the sputter glancing angle deposition (GLAD) technique. First type of nanostructured plates incorporates 600 nm long vertically aligned copper nanorod arrays grown with nanorod diameters and spacing varying between 50-100 and 20-100 nm, respectively. The second type incorporates 600 nm long tilted copper nanorod arrays grown with diameter values varying between 50-100nm and spacing in the range of 20-50 nm. Heat removal characteristics induced through jet impingement are investigated using the nanostructured plates and compared to the results obtained from a plain surface plate of copper thin film coated on silicon wafer surface. Heat generated by small scale electronic devices is simulated using four cylindrical aluminum cartridge heaters of 6.25 mm diameter and 31.75 mm length placed inside an aluminum base. Surface temperatures are recorded by a data acquisition system with four thermocouples integrated on the surface at various prescribed locations. Constant heat flux provided by the heaters is delivered to the nanostructured plate placed on top of the base. Volumetric flow rate and heat flux values are varied between 107.5-181.5 ml/min and 1-40 W/cm2 , respectively, in order to characterize the potential enhancement in heat transfer by nanostructured surfaces thoroughly. A single phase average heat transfer enhancement of 22.4% and a two phase average heat transfer enhancement of 85.3% has been realized using the nanostructured plate with vertical nanorods compared to flat plate. This enhancement is attributed to the increased heat transfer surface area and the single crystal property of the vertical Cu nanorods. On the other hand, nanostructured plate with tilted nanorods has shown poorer heat transfer performance compared to both the nanostructured plate with vertical nanorods and plain surface plate in the experiments performed. The lower heat transfer rate of the tilted Cu nanorods is believed to be due to the decreased supply of liquid jets to the base of the plate caused by their tilted orientation and closely spaced dense array structure. This leads to formation of air gaps that ultimately become trapped among the tilted nanorods, which results in reduced heat transfer surface area and increased resistance to heat transfer. In addition, non-single crystal structure of the tilted nanorods and resulting enhanced surface oxidation could further decrease their heat transfer performance. In the third study, a nanostructure based compact pool boiler cooling system consisting of an aluminum base housing the heaters, a pool and four different plates to change the surface texture of the pool is designed. Effects of nanostructured plates of different surface morphologies on boiling heat transfer performance of the system are studied. Three nanostructured plates featuring Si nanowires of diameter 850 nm and of three different lengths, 900 nm, 1800 nm and 3200 nm respectively, which are etched through single crystal p-type silicon wafers using metal assisted chemical etching (MaCE), are utilized to enhance the pool boiling heat transfer. A plain surface Si plate is used as the control sample. Constant heat flux is provided to the liquid within the pool on the surface of the aluminum base through the plate by boiling heat transfer. Existence of wall superheat gave rise to forming of vapor bubbles near the boiling temperature of the fluid, namely DI-Water. Bubbles emerged from the nanostructured plate along with the phase change. Nucleate boiling on the surface of the plate, bubble formation and bubble motion inside the pool created an effective heat removal mechanism from the heated surface to the liquid pool. Along with the enhancement in both boiling and single-phase region heat transfer coefficients, this study proves the ability of nanostructured plates in improving the performance of the cooling system

Optimisation of microchannels and micropin-fin heat sinks with computational fluid dynamics in combination with a mathematical optimisation algorithm

In recent times, high power density trends and temperature constraints in integrated circuits have led to conventional cooling techniques not being sufficient to meet the thermal requirements. The ever-increasing desire to overcome this problem has led to worldwide interest in micro heat sink design of electronic components. It has been found that geometric configurations of micro heat sinks play a vital role in heat transfer performance. Therefore, an effective means of optimally designing these heat sinks is required. Experimentation has extensively been used in the past to understand the behaviour of these heat extraction devices. Computational fluid dynamics (CFD) has more recently provided a more cost-effective and less time-consuming means of achieving the same objective. However, in order to achieve optimal designs of micro heat sinks using CFD, the designer has to be well experienced and carry out a number of trial-and-error simulations. Unfortunately, this will still not always guarantee an accurate optimal design. In this dissertation, a design methodology which combines CFD with a mathematical optimisation algorithm (a leapfrog optimisation program and DYNAMIC-Q algorithm) is proposed. This automated process is applied to three design cases. In the first design case, the peak wall temperature of a microchannel embedded in a highly conductive solid is minimised. The second case involves the optimisation of a double row micropin-fin heat sink. In this case, the objective is to maximise the total rate of heat transfer with the effect of the thermal conductivity also being investigated. The third case extends the micropin-fin optimisation to a heat sink with three rows. In all three cases, fixed volume constraint and manufacturing restraints are enforced to ensure industrial applicability. Lastly, the trends of the three cases are compared. It is concluded that optimal design can be achieved with a combination of CFD and mathematical optimisation.Dissertation (MEng)--University of Pretoria, 2011.Mechanical and Aeronautical EngineeringUnrestricte

Recent research developments in polymer heat exchangers: a review

Due to their low cost, light weight and corrosive resistant features, polymer heat exchangers have been intensively studied by researchers with the aim to replace metallic heat exchangers in a wide range of applications. This paper reviews the development of polymer heat exchangers in the last decade, including cutting edge materials characteristics, heat transfer enhancement methods of polymer materials and a wide range of polymer heat exchanger applications. Theoretical modelling and experimental testing results have been reviewed and compared with literature. A recent development, the polymer micro-hollow fibre heat exchanger, is introduced and described. It is shown that polymer materials do hold promise for use in the construction of heat exchangers in many applications, but that a considerable amount of research is still required into material properties, thermal performance and life-time behaviour

Surface Structure Enhanced Microchannel Flow Boiling

We investigated the role of surface microstructures in two-phase microchannels on suppressing flow instabilities and enhancing heat transfer. We designed and fabricated microchannels with well-defined silicon micropillar arrays on the bottom heated microchannel wall to promote capillary flow for thin film evaporation while facilitating nucleation only from the sidewalls. Our experimental results show significantly reduced temperature and pressure drop fluctuation especially at high heat fluxes. A critical heat flux (CHF) of 969 W/cm2 was achieved with a structured surface, a 57% enhancement compared to a smooth surface. We explain the experimental trends for the CHF enhancement with a liquid wicking model. The results suggest that capillary flow can be maximized to enhance heat transfer via optimizing the microstructure geometry for the development of high performance two-phase microchannel heat sinks.United States. Office of Naval Research (N00014-15-1-2483)Masdar Institute of Science & Technology - MIT Technology & Development Program (Cooperative agreement, Reference 02/MI/MI/CP/11/07633/GEN/G/00)United States. Air Force Office of Scientific ResearchBattelle Memorial InstituteSingapore-MIT Alliance for Research and Technology (SMART