Numerical investigation on the heat transfer and flow in the mini-fin heat sink for CPU

Fluid flow and heat transfer in the mini-rectangular fin heat sink for CPU of PC using de-ionized water as working fluid are numerically investigated. Based on the real PC operating conditions, the three-dimensional governing equations for fluid flow and heat transfer characteristics are solved using finite volume scheme. The standard k–ε turbulent model is employed to describe the flow structure and behavior. The predicted results obtained from the model are verified by the measured data. There is a reasonable agreement between the predicted results and experiments. The results of this study are expected to lead to guidelines that will allow the design of the cooling system with improved cooling performance of the electronic equipments increasing reliable operation of these devices.     

Experimental investigation of titanium nanofluids on the heat pipe thermal efficiency

The enhancement heat transfer of the heat transfer devices can be done by changing the fluid transport properties and flow features of working fluids. In the present study, therefore, the enhancement of heat pipe thermal efficiency with nanofluids is presented. The heat pipe is fabricated from the straight copper tube with the outer diameter and length of 15, 600 mm, respectively. The heat pipe with the de-ionic water, alcohol, and nanofluids (alcohol and nanoparticles) are tested. The titanium nanoparticles with diameter of 21 nm are used in the present study which the mixtures of alcohol and nanoparticles are prepared using an ultrasonic homogenizer. Effects of %charge amount of working fluid, heat pipe tilt angle and %nanoparticles volume concentrations on the thermal efficiency of heat pipe are considered. The nanoparticles have a significant effect on the enhancement of thermal efficiency of heat pipe. The thermal efficiency of heat pipe with the nanofluids is compared with that the based fluid.     

Effects of outlet port positions on the jet impingement heat transfer characteristics in the mini-fin heat sink

Effects of outlet port positions on the jet liquid impingement heat transfer characteristics in the mini-rectangular fin heat sink are numerically investigated. The three-dimensional governing equations for fluid flow and heat transfer characteristics are solved using finite volume scheme. The standard k-ε turbulent model is employed to solve the model for describing the heat transfer behaviors. The predicted results obtained from the model are verified by the measured data. The predicted results are reasonable agreement with the measured data. The outlet port positions have significant effect on the uniformities in velocity and temperature. Based on the results from this study, it is expected to lead to guidelines that will allow the design of the cooling system to ensure the electronic devices at the safe operating temperature.     

Experimental investigation of silver nano-fluid on heat pipe thermal performance

Nano-fluid is employed as the working medium for a conventional 211 μm wide × 217 μm deep grooved circular heat pipe. The nano-fluid used in this study is an aqueous solution of 35 nm diameter silver nano-particles. The experiment was performed to measure the temperature distribution and to compare the heat pipe thermal resistance using nano-fluid and DI-water. The tested nano-particle concentrations ranged from 1 mg/l to 100 mg/l. The condenser section of the heat pipe was attached to a heat sink that was cooled by water supplied from a constant-temperature bath maintained at 40 °C.At a same charge volume, the measured nano-fluid filled heat pipe temperature distribution demonstrated that the thermal resistance decreased 10–80% compared to DI-water at an input power of 30–60 W. The measured results also show that the thermal resistances of the heat pipe decrease as the silver nano-particle size and concentration increase.     

Thermal performance analysis and optimum design parameters of heat exchanger having perforated pin fins

This paper reports the heat transfer enhancement and corresponding pressure drop over a flat surface equipped with circular cross section perforated pin fins in a rectangular channel. The channel had a cross section area of 100–250 mm². The experiments covered the following ranges: Reynolds number 13500–42,000, clearance ratio (C/H) 0, 0.33 and 1 and interfin spacing ratio (S_y/D) 1.208, 1.524, 1.944 and 3.417. Correlation equations were developed for the heat transfer, friction factor and enhancement efficiency. The experimental results showed that the use of circular cross section pin fins may lead to heat transfer enhancement. Enhancement efficiencies varied between 1.4 and 2.6 depending on clearance ratio and interfin spacing ratio. Using a Taguchi experimental design method, optimum design parameters and their levels were investigated. Nusselt number and friction factor were considered as performance parameters. An L₉(3³) orthogonal array was selected as an experimental plan. First of all, each goal was optimized separately. Then, all the goals were optimized together, considering the priority of the goals, and the optimum results were found to be Reynolds number of 42,000, fin height of 50 mm and streamwise distance between fins of 51 mm.     

Heat transfer in rectangular microchannels heat sink using nanofluids

The effect of using nanofluids on heat transfer and fluid flow characteristics in rectangular shaped microchannel heat sink (MCHS) is numerically investigated for Reynolds number range of 100–1000. In this study, the MCHS performance using alumina–water (Al₂O₃-H₂O) nanofluid with volume fraction ranged from 1% to 5% was used as a coolant is examined. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using finite volume method. The MCHS performance is evaluated in terms of temperature profile, heat transfer coefficient, pressure drop, friction factor, wall shear stress and thermal resistance. The results reveal that when the volume fraction of nanoparticles is increased under the extreme heat flux, both the heat transfer coefficient and wall shear stress are increased while the thermal resistance of the MCHS is decreased. However, nanofluid with volume fraction of 5% could not be able to enhance the heat transfer or performing almost the same result as pure water. Therefore, the presence of nanoparticles could enhance the cooling of MCHS under the extreme heat flux conditions with the optimum value of nanoparticles. Only a slight increase in the pressure drop across the MCHS is found compared with the pure water-cooled MCHS.     

Experimental investigation of heat transfer and turbulent flow friction in a tube fitted with perforated conical-rings

Perforated conical-ring (PCR) is one of the turbulence-promoter/turbulator devices for enhancing the heat transfer rate in a heat exchanger system. In the present paper, the influences of the PCR on the turbulent convective heat transfer (Nu), friction factor (f) and thermal performance factor (η) characteristics have been investigated experimentally. The perforated conical-rings (PCRs) used are of three different pitch ratios (PR = p/D = 4, 6 and 12) and three different numbers of perforated holes (N = 4, 6 and 8 holes). The experiment conducted in the range of Reynolds number between 4000 and 20,000, under uniform wall heat flux condition and using air as the testing fluid. The experimental results obtained by using the plain tube and the tube equipped with the typical conical-ring (CR) are also reported for comparison. It is found that the PCR considerably diminishes the development of thermal boundary layer, leading to the heat transfer rate up to about 137% over that in the plain tube. Evidently, the PCRs can enhance heat transfer more efficient than the typical CR on the basis of thermal performance factor of around 0.92 at the same pumping power. Over the range investigated, the maximum thermal performance factor of around 0.92 is found at PR = 4 and N = 8 holes with Reynolds number of 4000.     

Heat transfer enhancement of finned-tube heat exchanger using nozzle- and diffuser-shaped fins instead of straight fins

In this paper, a new method has been used to improve the heat transfer rate in the finned-tube heat exchanger with nozzle- and diffuser-shaped arrangement. For this study, the effect of several parameters was studied numerically. For the computational fluid dynamics simulation, the continuity, momentum, and energy equations were solved by the finite volume method using the standard k–ԑ model. The rate of heat transfer increases with the decreasing of fin bend radius (15 < R_fb < 20) for both nozzle-shaped fin and diffuser-shaped fin. By increasing of side temperature (600 < T_side < 900) and side Reynolds number (2000 < Re_side < 5000) the heat transfer rate increased for both nozzle- and diffuser-shaped fins. Results showed that a nozzle-shaped fin with a fin bend radius of 15 mm under the condition of Re_in = 20,000, T_side = 900 K, and Re_side = 3400 has a higher effect on heat transfer in comparison with the other types of fins. The maximum heat transfer rate was almost 39% and 35% for the nozzle-shaped fin with a bend radius of 15 mm and diffuser-shaped fin with a bend radius of 15 mm compared with the simple tube, respectively. Finally, correlational equations have been suggested to forecast the average Nu number as functions of various parameters of the tube equipped with different types of outer fins involving nozzle- and diffuser-shaped.     

Compact cooler for electronics on the basis of a pulsating heat pipe

The paper presents the results of developing and investigating a compact cooler for electronics made on the basis of a closed loop pulsating heat pipe (CLPHP). The cooler is made of a copper tube 5.6 m long with OD of 2 mm and ID of 1.2 mm in the form a 3D spiral containing 17 turns. The device is equipped with a light copper radiator with a finning area of 1670 cm², which was blown by an axial fan located inside the spiral. The thermal interface of the cooler situated in the heating zone is made of a copper plate with a thermocontact surface measuring 40 × 35 mm, which was in thermal contact with all the turns of the device. The cooler overall dimensions are 105 × 100 × 60 mm, its mass is 350 g.The operation of the cooler has been investigated with water, methanol and R141b as working fluids at a uniform and concentrated supply of a heat load in different heating modes. A reliable operation of the device has been demonstrated in the range of heat loads from 5 to 250 W. A minimum thermal resistance “heat source–ambient air” equal to 0.32 °C/W was attained with water and methanol as working fluids at a uniform heat load of 250 W. With a heat load concentrated on a section of the thermal interface limited by an area of 1 cm², a minimum value of thermal resistance equal to 0.62 °C/W was attained at a heat load of 125 W when methanol was used as a working fluid.     

Experimental investigation of a flat plate heat pipe performance using IR thermal imaging camera

This paper presents results and analysis of an experimental investigation into determining the thermal performance of a flat plate heat pipe using infra red (IR) thermal imaging camera. Steady state and transient temperature distribution of the evaporator surface of the flat plate heat pipe were measured using a single heat source with varied heat flux inputs. For performance comparison, the experimental measurements were also carried out on an identical flat plate heat pipe with a defect and on a solid copper block of similar dimensions. It was shown that temperature excursion on the surface of the fully functioning flat plate heat pipe is less than 3 °C for operating temperatures up to 90 °C and heat flux inputs ranging from 4 to 40 W/cm². Furthermore, the thermal spreading resistance of the flat plate heat pipe was found to be about 40 times smaller than that of the solid copper block and flat plate heat pipe with a defect.     

3D numerical optimization of a heat sink base for electronics cooling

In this paper, the possible optimal thickness of a heat sink base has been explored numerically with different convective heat transfer boundary conditions in a dimensionless three dimensional heat transfer model. From the numerical results, relations among different heat transfer mechanisms (natural or forced, air or liquid), different area ratios of a heat sink to a heating source, and the lowest thermal resistance have been obtained and discussed. Also a simple correlation for these three parameters from data fitting is given for guiding a heat sink design.     

Investigating the effect of various fin geometries on the thermal performance of a heat sink under natural convection

Experimentally investigates heat dissipation by different longitudinal fins fitted to a cylindrical heat sink under natural convection conditions. Five aluminum fin configurations at base temperatures (70°C, 85°C, 100°C, and 115°C) were studied. The first fin was plain (fin1), while second fin had a triangular edge (fin2). The rest fins have the same triangular edge but with six 1cm circular perforations near the edge (fin3). While the perforations in fin4 were in the middle longitudinal fin length. The last fin (fin5) had twelve 0.5 cm circular perforations distributed into two columns. The measurements were validated with theoretical correlation with an acceptable deviation. The results showed that fin2, fin3, fin4, and fin5 dissipate more heat by 2.4%, 8.7%, 11.4%, and 5% than the flat fin with 9.8%, 11.85%, 11.85%, and 10.82% weight reduction, respectively. The heat transfer coefficient enhanced by 7.98%, 16.81%, 12.35%, and 5.44% for fin5, fin4, fin3, and fin2, respectively. Large circular perforation was more effective to dissipate heat especially when located near the heat source as in fin4 which gives the best heat dissipation with more weight reduction. The proposed fins efficiency were greater than 92%.     

Cooling performance of a microchannel heat sink with nanofluids

In this paper, the cooling performance of a microchannel heat sink with nanoparticle–fluid suspensions (“nanofluids”) is numerically investigated. By using a theoretical model of thermal conductivity of nanofluids that accounts for the fundamental role of Brownian motion, we investigate the temperature contours and thermal resistance of a microchannel heat sink with nanofluids such as 6 nm copper-in-water and 2 nm diamond-in-water. The results show that the cooling performance of a microchannel heat sink with water-based nanofluids containing diamond (1 vol.%, 2 nm) at the fixed pumping power of 2.25 W is enhanced by about 10% compared with that of a microchannel heat sink with water. Nanofluids reduce both the thermal resistance and the temperature difference between the heated microchannel wall and the coolant. Finally, the potential of deploying a combined microchannel heat sink with nanofluids as the next generation cooling devices for removing ultra-high heat flux is shown.     

Experimental investigation of convective heat transfer and pressure loss in a round tube fitted with circular-ring turbulators

This paper presents the effect of the circular-ring turbulator (CRT) on the heat transfer and fluid friction characteristics in a heat exchanger tube. The experiments were conducted by insertion of CRTs with various geometries, including three different diameter ratios (DR = d/D = 0.5, 0.6 and 0.7) and three different pitch ratios (PR = p/D = 6, 8 and 12). During the test air at 27 °C was passed through the test tube which was controlled under uniform wall heat flux condition. The Reynolds number was varied from 4000 to 20,000. According to the experimental results, heat transfer rates in the tube fitted with CRTs are augmented around 57% to 195% compared to that in the plain tube, depending upon operating conditions. In addition, the results also reveal the CRT with the smallest pitch and diameter ratios offers the highest heat transfer rate in accompany with the largest pressure loss.     

Experimental investigation of aluminum oxide nanofluid on heat pipe thermal performance

In this research, the effect of using aluminum oxide nanofluid (pure water mixed with Al₂O₃ nanoparticle with 35 nm diameter) on the thermal efficiency enhancement of a heat pipe on the different operating state was investigated.     

Laminar convective heat transfer in a microchannel with internal longitudinal fins

A numerical study was conducted to investigate the fluid flow and heat transfer characteristics of a square microchannel with four longitudinal internal fins. Three-dimensional numerical simulations were performed on the microchannel with variable fin height ratio in the presence of a hydrodynamically developed, thermally developing laminar flow. Constant heat flux boundary conditions were assumed on the external walls of the square microchannel. Results of the average local Nusselt number distribution along the channel length were obtained as a function of the fin height ratio. The analysis was carried out for different fin heights and flow parameters. Interesting observations that provide more physical insight on this passive enhancement technique, and the existence of an optimum fin height are brought out in the present study.     

Optimum thermal performance of microchannel heat sink by adjusting channel width and height

A three-dimensional numerical model of the microchannel heat sink is presented to study the effects of heat transfer characteristics due to various channel heights and widths. Based on the theory of a fully developed flow, the pressure drop in the microchannel is derived under the requirement of the flow power for a single channel. The effects of two design variables representing the channel width and height on the thermal resistance are investigated. In addition, the constraint of the same flow cross section is carried out to find the optimum dimension. Finally, the minimum thermal resistance and optimal channel width with various flow powers and channel heights are obtained by using the simulated annealing method.     

Numerical investigation of a PCM-based heat sink with internal fins

The present study explores numerically the process of melting of a phase-change material (PCM) in a heat storage unit with internal fins open to air at its top. Heat is transferred to the unit through its horizontal base, to which vertical fins made of aluminum are attached. The phase-change material is stored between the fins. Its properties used in the simulations, including the melting temperature of 23-25 °C, latent and sensible specific heat, thermal conductivity and density in solid and liquid states, are based on a commercially available paraffin wax.A detailed parametric investigation is performed for melting in a relatively small system, 5-10 mm high, where the fin thickness varies from 0.15 mm to 1.2 mm, and the thickness of the PCM layers between the fins varies from 0.5 mm to 4 mm. The ratio of the PCM layer to fin thickness is held constant. The temperature of the base varies from 6 °C to 24 °C above the mean melting temperature of the PCM.Transient three- and two-dimensional simulations are performed using the Fluent 6.0 software, yielding temperature evolution in the fins and the PCM. The computational results show how the transient phase-change process, expressed in terms of the volume melt fraction of the PCM, depends on the thermal and geometrical parameters of the system, which relate to the temperature difference between the base and the mean melting temperature, and to the thickness and height of the fins.In search for generalization, dimensional analysis of the results is performed and presented as the Nusselt numbers and melt fractions vs. the Fourier and Stefan numbers and fin parameters. In some cases, the effect of Rayleigh number is significant and demonstrated.     

Experimental investigation of the transient thermal performance of a bent heat pipe with grooved surface

A bent copper–water heat pipe with grooved inner surface has been investigated experimentally. A comparison between the bent and the straight heat pipes was performed at different inclination angle. Experimental results show that there is a small temperature difference between the condenser of the straight and that of the bent at the vertical orientation. The temperature difference increases as an inclination angle increases. Furthermore, the response time increases as the inclination angle increases. The thermal response of the straight to a sudden heat load is slightly faster than that of the bent. However, as the inclination angle increases to after the horizontal, the heat flux at the condensers decreases nonlinearly and the response time increases nonlinearly. A two-phase flow map has been proposed to explain the nonlinear performance of the thermal response and the heat flux, based on force balance among gravity, capillary, friction and buoyancy force acting on the working fluids. The nonlinear performance of the thermal response and the heat flux results from the capillary blocking due to formation of liquid bridge of two-phase flow. It was also found that the bent heat pipe is more sensitive to the change of the inclination angle than the straight in terms of the thermal response time and the heat flux of the condenser. The heat flux of the bent decreases faster than that of the straight after the horizontal orientation.     

Method to improve geometry for heat transfer enhancement in PCM composite heat sinks

Use of composite heat sinks (CHS), constructed using a vertical array of ‘fins’ (or elemental composite heat sink, ECHS), made of large latent heat capacity phase change materials (PCM) and highly conductive base material (BM) is a much sought cooling method for portable electronic devices, which are to be kept below a set point temperature (SPT). This paper presents a thermal design procedure for proper sizing of such CHS, for maximizing the energy storage and the time of operation until all of the latent heat storage is exhausted.For a given range of heat flux, q″, and height, A, of the CHS, using a scaling analysis of the governing two dimensional unsteady energy equations, a relation between the critical dimension for the ECHS and the amount of PCM used (?) is determined. For a ?, when the dimensions of the ECHS are less than this critical dimension, all of the PCM completely melts when the CHS reaches the SPT. The results are further validated using appropriate numerical method solutions. A proposed correlation for chosen material properties yields predictions of the critical dimensions within 10% average deviation. However, the thermal design procedure detailed in this paper is valid, in general, for similar finned-CHS configurations, composed of any high latent heat storage PCM and high conductive BM combination.