Effect of non-uniform heating on the performance of the microchannel heat sinks

The present study experimentally investigates the performance of a 2-pass microchannel heat sink subject to non-uniform heating. The size of the microchannel heat sink is 132 mm × 82 mm × 6 mm with a rectangular channel of 1 mm × 1 mm. Three independent heaters having identical size (96 mm × 38.5 mm × 1 mm) is placed consecutively below the microchannel heat sink. Two kinds of manifolds are used for testing of the microchannel, one with a side entrance (type A) and the other with a front entrance (type B). Test results show that both maximum temperature and average temperature rise with the total input power, and this is applicable for both manifolds. For uniform heating condition, the maximum temperature for type B manifold is much lower than that for type A manifold due to a better flow distribution and heat transfer performance. The pressure drop is slightly reduced with the rise of supplied power. For non-uniform heating, the maximum temperature and the average temperature depend on the location of heaters. For the same supplied power with non-uniform heating, it is found that heater being placed at the inlet of the microchannel will give rise to a higher maximum temperature than that being placed at the rear of the heat sink. This phenomenon is especially pronounced when the inlet flowrate is comparatively small and becomes less noted as the inlet flowrate is increased to 0.7 L/min.     

Visualization of flow patterns and bubble behavior during flow boiling in open microchannels

Flow boiling in microchannels is favored by the heat transfer community due to the high heat transfer rates that can be obtained with lower mass flow rates. However, the heat transfer rates for flow boiling in microchannels are impacted by flow reversals and flow instabilities. An open microchannel structure was recently proposed to reduce the impact of the flow boiling instabilities. Subcooled flow boiling experiments were conducted in open microchannels using deionized water. The open microchannels had 6 parallel channels with a 0.3 mm uniform thickness gap above them The channels were fabricated on a 6 mm × 40 mm copper block. The channels were 0.5 mm wide and 0.3 mm deep with 0.43 mm wide fins between them. Flow visualizations were performed with a high-speed CCD camera with the results showing that the flow regimes in the open microchannels differ from those in closed microchannels with stratified flow and no flow instability. Two types of confined bubbles were observed with characterizations of the effects of the bubbles on each other. The heat transfer mechanisms for flow boiling in open microchannels are also described.     

Analysis of flow patterns emerging during evaporation in parallel microchannels

The evaporation processes of 2-propanol and water in cyclo olefin polymer (COP) and silicon microchannels of square cross-section are studied with a high-speed camera. The COP channels with a cross-section of 50 μm × 50 μm are rather smooth, whereas the 30 μm × 30 μm silicon channels have comparatively rough surfaces. For the COP channels, two different evaporation modes are identified, both with oscillating liquid–vapor menisci. One of these modes is characterized by an extremely rapid evaporation and a corresponding discontinuous shift of the meniscus. In the silicon channels four different evaporation modes are observed. Oscillatory motion of the liquid fronts also dominates here, and depending on the total mass flow and the wall temperature the oscillations in different channels are synchronized or desynchronized. Besides the flow patterns also the velocity trajectories of the evaporating liquid fronts are analyzed in detail and show a rather good reproducibility over different channels and different cycles. Compared to most other studies reported in this field, bubble nucleation is found to be of secondary importance for the evaporation processes.     

Heat transfer during condensation of moving steam in a narrow channel

The paper presents the results of experimental investigation of heat transfer and hydrodynamics during condensation of moving steam in a narrow channel of square cross-section 2 mm × 2 mm. The channel had a serpentine shape, the channel length was 660 mm. An experimental cell simulated conditions of heat transfer in the condenser of loop heat pipes. The steam velocity at the channel inlet ranged from 13 to 52 m/s, the pressure was 1 atm. The temperature of the cooling water varied from 70 to 95 °C. The annular flow pattern was noted in the whole range of the regime parameters. There was a clear boundary between the condensation zone and the zone occupied by the condensed phase downstream. Temperature has measured along the channel, and the heat-transfer coefficients have been determined. The coefficient values varied from 10,000 to 55,000 W/K m² depending on the steam velocity at the channel inlet and the cooling temperature. The efficiency of the condenser – heat exchanger has been investigated.     

Redirection of sunlight by microstructured components – Simulation,fabrication and experimental results

This paper presents a non-tracking microstructured light redirecting device, which can be integrated into architectural glass. When fixed in the upper area of a window above eye level it redirects the light from solar altitudes between 15° and 65° and illuminates a room without causing glare.Ray-tracing calculations are employed as a tool for identifying suitable configurations and geometries. The results of the simulations show the advantage of combinations of lens-like with prism-like geometries in comparison to conventional microprism arrays regarding the overall light redirection efficiency as well as the producibility. The redirecting device is more lightweight, gives better integration options and is producible in a more economic manufacturing process as systems with similar performance. Measurements of cast silicone prototypes (100 mm × 100 mm × 4 mm) confirmed the simulation results. By now the performance has also been shown by large scale industrially produced acrylic panels with dimensions of 1500 mm × 400 mm × 4 mm.     

Experimental investigation of buoyant flows in inclined differentially heated cavities

This experimental investigation focuses on the effects of angle of inclination on buoyancy-driven flows inside tall, rectangular, differentially-heated cavities. It considers a rectangular cavity with an aspect ratio of 28.6, with its two long sides maintained at different temperatures and the two short, end-walls, thermally insulated. The spanwise aspect ratio is 6.82 and the side walls are also thermally insulated. The Rayleigh number, based on the temperature difference and spacing between the long sides, is 0.86 × 10⁶ for most cases and the working fluid is air (Prandtl number 0.71). Experimental data, for the flow and the thermal fields, using laser Doppler anemomentry and thermocouple traverses respectively, are presented for the cavity inclined at 60° and 15° to the horizontal, for both stable (the hot surface being the upper surface) and unstable orientations. The 15° stable case is investigated at a higher Rayleigh number of 1.54 × 10⁶ and some additional data for the 15° unstable case are also presented at this higher value of Rayleigh number. For moderate angles of inclination, the flow is two-dimensional and the effects of inclination are primarily confined to the fluctuating fields. For large angles of inclination, the flow becomes three-dimensional. In the unstable 15° angle of inclination case a set of four longitudinal vortices are formed over the entire length of the cavity, with four counter-rotating re-circulation cells within the cross-section parallel to the thermally active walls. The stable 15° angle of inclination leads to the formation of two longitudinal vortices and two re-circulation cells. At the 15° angle (stable and unstable), the enhanced mixing leads to uniform temperature in the cavity core and thus to only minor deviations from two-dimensionality in the thermal field. A modest rise in Rayleigh number, in the 15° unstable case, does not affect the mean motion, but causes an increase in the normalised turbulence intensities.     

Numerical and experimental thermal analysis for a metallic hollow cylinder subjected to step-wise electro-magnetic induction heating

In this study, basic electro-magnetic and heat transfer theories were applied to simulate the electro-magnetic and temperature fields in a steel hollow cylinder subjected to step-wise induction heating from outside. Three different sizes (Pipe A, D_o × D_i × L = 95 mm × 29 mm × 1000 mm, Pipe B, D_o × D_i × L = 110 mm × 39 mm × 1120 mm, Pipe C, D_o × D_i × L = 131 mm × 47 mm × 1450 mm) of the workpieces were numerically and experimentally investigated and compared. The temperatures on the inside and outside surface of the workpiece during the induction heating process were measured by thermocouples and an infrared thermal imaging system, respectively. The applied power input is a steep-wise function (constant high power, 0–8 min, and decrease to it 60%, 8–12 min, and then increase it original high power, 12–20 min). The process of induction heating heats the hollow cylinder from ambient temperature above the Curie point. It is shown that the inside temperature of the hollow cylinder is below the outside temperature initially (0–8 min), and then a constant temperature is held for approximately 4 min and finally the inside temperature is higher than the outside temperature. The numerical results agreed with the experimental data within 15%. The numerical simulation of three different air gaps (5 mm, 15 mm and 25 mm) between the coil and the workpiece were also performed. It is found that the temperature is increased as the air gap is decreased. The average temperatures of the hollow steel for air gap = 5 mm are 10 °C and 15 °C higher those for air gap = 15 mm, 25 mm, respectively.     

Investigation of the evacuation pressure on the performance of pulsating heat pipe

This study examines the influence of evacuation pressure on the startup and overall performance of pulsating heat pipes (PHP). The outer diameter of the copper tube is 3 mm and 2 mm (double tube design) having a wall thickness of 0.3 mm and 0.2 mm, respectively. The working fluids in this study include water and HFE-7000 with filling ratios around 50%, and the evacuation pressure ranging from 0.01 Torr to atmosphere pressure. For an evacuation pressure of 0.01 Torr at a supplied power of 80 W, the thermal resistance of the PHP filling with water is 0.928 K/W while it is 1.161 K/W for HFE-7000. However, the trend is reversed and the thermal resistance for HFE-7000 is lower than water when the evacuation pressure is increased over 100 Torr. The corresponding effective thermal conductivity of water-filling PHP reaches 51,448 W/m K in comparison with 12,692 W/m K for HFE-7000. However, the effective thermal conductivity for water-filling PHP drops appreciably with rising evacuation pressure, and the PHP is not functional at the atmosphere pressure. Conversely, although the effective thermal conductivity for HFE-7000 PHP still drops with the rise of evacuation pressure, the HFE-7000 PHP is still in operation even without any evacuation. The gigantic difference in the startup and performance of PHP is related to the solubility of non-condensable gas amid water and HFE-7000.     

Instability phenomena in a two-phase microchannel thermosyphon

The behavior of a two-phase thermosyphon, consisting of a microchannel evaporator plate and a condenser, is investigated to gain insight into the system limiting instability. A microchannel plate has been fabricated with 56 square channels that have a 1 × 1 mm cross section and a length of 115 mm. Experiments have been conducted for various condenser heights with the heat flux as the control variable. A step increase in heat flux is used to quantify the response of the system, including variations in mass flow rate, temperature, and pressure drop. Results show that small fluctuations about the steady state give rise to the instability for situations with a uniform heat load. A predictive model based on the momentum equation is introduced to estimate the onset of instability, and the threshold heat flux is predicted to within ±10% uncertainty.     

Experimental investigations on phase change material based finned heat sinks for electronic equipment cooling

This paper reports the results of an experimental investigation of the performance of finned heat sinks filled with phase change materials for thermal management of portable electronic devices. The phase change material (PCM) used in this study is n-eicosane and is placed inside a heat sink made of aluminium. Aluminium acts as thermal conductivity enhancer (TCE), as the thermal conductivity of the PCM is very low. The heat sink acts as an energy storage and a heat-spreading module. Studies are conducted for heat sinks on which a uniform heat load is applied for the unfinned and finned cases. The test section considered in all cases in the present work is a 80 × 62 mm² base with TCE height of 25 mm. A 60 × 42 mm² plate heater with 2 mm thickness is used to mimic the heat generation in electronic chips. Heat sinks with pin fin and plate fin geometries having the same volume fraction of the TCE are used. The effect of different types of fins for different power level (ranging from 2 to 7 W) in enhancing the operating time for different set point temperatures and on the duration of latent heating phase were explored in this study. The results indicate that the operational performance of portable electronic device can be significantly improved by the use of fins in heat sinks filled with PCM.     

Investigation of natural circulation in cavities with uniform heat generation for different Prandtl number fluids

Natural convection in enclosures with uniform heat generation and isothermal side walls is studied here. For the rectangular enclosure, two-dimensional conservation equations are solved using SIMPLE algorithm. Parametric studies are conducted to examine the effects of orientation of the cavity, fluid properties (Pr number), and aspect ratio for Rayleigh numbers up to 10⁶. For a horizontal square cavity, the flow becomes periodically oscillating at Ra = 5 × 10⁴ and chaotic at Ra = 8 × 10⁵. With a slight increase in the inclination angle, the oscillations die and for inclination angles greater than 15⁰, the flow attain a steady state over a range of Ra. It is found that for tall cavities (aspect ratio > 1), the steady-state solution is obtained for all values of Ra considered here. However, for wide cavities (aspect ratio < 1), an oscillatory flow regime is observed. The maximum temperature within the cavity is calculated for the range of Ra, aspect ratio and Pr number. Correlations for the maximum cavity temperature is presented here. The values of critical Rayleigh number at which the convection sets in the rectangular cavity are also studied and two distinct criteria are determined to evaluate the critical Rayleigh number. Further, a three-dimensional simulation is performed for a cubic cavity. It is found that the steady state solutions are obtained for all Rayleigh number, except at Ra = 10⁶. This is in contrast to the predictions for a two-dimensional square cavity, which has an oscillatory zone from Ra = 5 × 10⁴ onwards.     

Transmission of near-limit detonation wave through a planar sudden expansion in a narrow channel

Transmission of single-cell and spinning detonation waves in C₂H₄ + 3(O₂ + βN₂) mixtures through a 2-D sudden expansion experimentally studied using high-speed cinematography and soot film visualization. Nitrogen dilution ratio, β, is utilized to control cell size and detonation mode. Detonation wave of ethylene/oxygen/nitrogen mixture was initiated via DDT in the 1 mm × 1 mm cross-section and 250 mm long initiator channel before propagating into the 3 mm × 1 mm receptor channel. Visualizations show that detonation waves were extinct and accompanied with abrupt decrease in visible reaction front propagating velocities right after passing through the sudden expansion. However, re-acceleration of the reaction front and re-initiation of the detonation wave were observed downstream in the expanded receptor section. Two re-initiation modes with large disparity in the re-initiation distance were experimentally characterized. For mixtures with nitrogen dilution ratio equals 0.3 or less, the cellular detonation front propagated with single cell in the initiator section before entering the sudden expansion. The re-initiation distance was less than 50 mm and was likely achieved via shock reflection. Velocity characterization shows that steady propagating speed of the detonation wave is ～100 m/s higher in receptor section than in the initiator section. Since the cell size became larger than 1 mm for mixtures with β ? 0.3, the detonation wave propagated in spinning detonation mode before transmitting into the expanded section. The reaction front would have to go through another DDT process to reach detonation state in the receptor section, and the re-initiation distance was increased to more than 150 mm. Moreover, step height of the sudden expansion was proposed as the characteristic length scale to obtain a unified non-dimensional correlation between re-initiation distance and detonation cell size.     

An experimental investigation on the thermal efficiency of fractal tree-like microchannel nets

In this paper, a fractal tree-like microchannel net heat sink (20 mm × 20 mm × 1.4 mm) for cooling of electronic chips was fabricated on a silicon wafers by advanced MEMS technology. The length, width and height of the entrance microchannel were 10 mm, 800 μm and 25 μm, respectively. The fractal dimension D and the circulation number m of the fractal tree-like microchannel net were 2 and 4, respectively. It is confirmed experimentally that the thermal efficiency (defined as heat transfer rate per unit power required) of such a fractal tree-like microchannel heat sink is much higher than that of the traditional parallel microchannel heat sink for the same heat transfer rate, the same temperature difference and the same inlet velocity.     

Efficient spreaders for cooling high-power computer chips

Investigated is the performance of composite spreaders, consisting of a top layer of porous graphite (⩾0.4 mm), for enhanced cooling by nucleate boiling of FC-72 dielectric liquid, and a copper substrate (⩽1.6 mm), for efficient spreading of the dissipated thermal power by the underling 10 × 10 mm or 15 × 15 mm high-power computer chips. The analysis assumes uniform thermal power dissipation by the chips and calculates the square surface area of the spreader, along with the spreading, boiling and total thermal resistances, the maximum chip temperature, and the removed thermal power from the spreader surface by saturation or subcooled nucleate boiling of FC-72 liquid. These performance parameters are determined as functions of the thickness of the copper substrate and the size of the underlying chip. When compared with those of copper and porous graphite spreaders of the same total thickness, 2.0 mm, the performance of the composite spreaders is superior for cooling high-power computer chips. When cooled by nucleate boiling of 30 K subcooled FC-72 liquid, the composite spreader removes 160.3 W and 98.4 W of dissipated thermal power by the underlying 10 × 10 mm and 15 × 15 mm chips, at total thermal resistances of 0.29 and 0.48 °C/W. When the same spreader is cooled by saturation boiling of FC-72, the removed thermal power decreases to 85.6 W and 53.4 W, and the total thermal resistances also decrease to 0.12 and 0.20 °C/W, respectively. Although the calculated surface temperatures of the chips are not uniform, the maximum temperatures are <71 °C and the temperature differential across the chips is <8 °C. For the same cooling condition, the calculated surface area of the copper spreaders, the total thermal resistance, and the maximum chip temperature are much higher, but the removed thermal powers from the surface of spreaders are much lower than with composite spreaders. The calculated surface areas of the porous graphite spreaders are smaller, the thermal powers removed from surface of these spreaders are much lower and both the total thermal resistance and the maximum chip temperature are higher than those with composite spreaders.     

Effects of duct inclination angle on thermal entrance region of laminar and transition mixed convection

Experimental investigation was performed on the mixed convection heat transfer of thermal entrance region in an inclined rectangular duct for laminar and transition flow. Air flowed upwardly and downwardly with inclination angles from ?90° to 90°. The duct was made of duralumin plate and heated with uniform heat flux axially. The experiment was designed for determining the effects of inclination angles on the heat transfer coefficients and friction factors at seven orientations (θ = ? 90°, ?60°, ?30°, 0°, 30°, 60° and 90°), six Reynolds numbers (Re ≈ 420, 840, 1290, 1720, 2190 and 2630) within the range of Grashof numbers from 6.8 × 10³ to 4.1 × 10⁴. The optimum inclination angles that yielded the maximum heat transfer coefficients decreased from 30° to ?30° with the increase of Reynolds numbers from 420 to 1720. The heat transfer coefficients first increased with inclination angles up to a maximum value and then decreased. With further increase in Reynolds numbers, the heat transfer coefficients were nearly independent of inclination angles. The friction factors decreased with the increase of inclination angles from ?90° to 90° when Reynolds numbers ranged from 420 to 1290, and independent of inclination angles with higher Reynolds numbers.     

Dry-out CHF correlation for R134a flow boiling in a horizontal helically-coiled tube

An experimental study was carried out to investigate the R134a dry-out critical heat flux (CHF) characteristics in a horizontal helically-coiled tube. The test section was heated uniformly by DC high-power source, and its geometrical parameters are the outer diameter of 10 mm, inner diameter of 8.4 mm, coil diameter of 300 mm, helical pitch of 75 mm and valid heated length of 1.89 m. The experimental parameters are the outlet pressures of 0.30–0.95 MPa, mass fluxes of 60–500 kg m^?2 s^?1, inlet qualities of ?0.36–0.35 and heat fluxes of 7.0 × 10³–5.0 × 10⁴ W m^?2. A method based on Agilent BenchLink Data Logger Pro was developed to determine the occurrence of CHF with a total of 68 T-type thermocouples (0.2 mm) set along the tube for accurate temperature measurement. The characteristics of wall temperatures and the parametric effect on dry-out CHF showed that temperature would jump abruptly at the point of CHF, which usually started to form at the front and offside (270° and 90°) of the outlet cross-section. The CHF values decrease nearly linearly with increasing inlet qualities, while they decrease more acutely with increasing critical qualities, especially under larger mass flux conditions. The mass flux has a positive effect on CHF enhancement, but the pressure has negative one. A new dimensionless correlation was developed to estimate dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes under current experimental conditions and compared to calculated results from Bowring and Shah correlations.     

A Bayesian approach for the simultaneous estimation of surface heat transfer coefficient and thermal conductivity from steady state experiments on fins

This paper explores the potential of the Bayesian approach to estimate multiple parameters from simple, inexpensive experiments on natural convection heat transfer from a vertical rectangular fin. A vertical fin of rectangular cross-section is placed on two heated horizontal aluminium blocks of size 250 × 75 × 10 (all in mm) that act as a horizontal base and hold the extended surface made up of mild steel of size 250 × 160 × 4 (all in mm). A heater is placed below the aluminium block and to restrict the heat flow, the bottom side of the heater is insulated with glass wool. Steady state experiments are carried out to obtain the temperature distribution for different levels of heating and temperatures on the fin are recorded with K-type thermocouples. Using data from these experiments, two critical parameters namely, the average heat transfer coefficient and the thermal conductivity of the extended surface are first individually and later simultaneously obtained using the Bayesian approach thereby obviating the need for sophisticated equipment. The above two parameters are correlated thereby making their simultaneous estimation very challenging. The Markov Chain Monte Carlo (MCMC) is used for the estimation without and with subjective priors on the two parameters. The uncertainties are obtained explicitly in the form of standard deviation. The addition of subjective priors is the hallmark of the Bayesian approach as it reduces the standard deviation of the estimates. This considerably helps regularizing the ill-posedness and becomes a necessity for estimating correlated parameters. A discussion on the optimum number of temperature measurements needed for estimating the parameters with a given accuracy using the Bayesian method is also presented.     

Convection and instability of thermocapillary flow in a liquid bridge subject to a non-uniform rotating magnetic field

A three-dimensional liquid bridge is considered in this study to numerically investigate the effects of an external non-uniform rotating magnetic field (RMF) on the thermocapillary flow in semiconductor melt under microgravity. Simulations are carried out to examine the convection and instability features of the thermocapillary flow over a range of Marangoni numbers (Ma = 15–50) under a non-uniform RMF. The present results show that applying an external non-uniform RMF enhances the maximum tangential velocity and depresses the maximum axial velocity. As a consequence, an approximately axisymmetric flow is maintained in the melt under the effect of the non-uniform RMF, which is beneficial for growing high quality crystal. Further investigation of the thermocapillary flow subject to different non-uniform RMFs (corresponding to Taylor numbers Ta = 3.8 × 10²–1.86 × 10⁴ and Rotating Reynolds number Re_ω = 2.2 × 10⁴) reveals that the thermocapillary convection may undergo a transition from the approximately axisymmetric steady flow to a periodically oscillatory flow for Ma above a critical value. The critical Ma generally increases with the intensity of the non-uniform RMF.     

Visualization study of pool boiling from thin confined enhanced structures

A visualization study of pool boiling at atmospheric pressure from top-covered enhanced structures was conducted for a dielectric fluorocarbon liquid (PF 5060). The single layer enhanced structures studied were fabricated in copper and quartz, had an overall size of 10 mm × 10 mm and were 1 mm thick. The parameters investigated in this study were the heat flux (in the range of 1–11 W/cm² for copper and 1–4 W/cm² for quartz) and the width of the microchannels (65–360 μm). A high-speed camera (maximum frame rate 1000 f/s at full resolution) with attached magnifying lens allowed precise observation of the evaporation process in the bottom and top channels. The heat transfer performance of the enhanced structures was found to depend weakly on the channel width. The internal evaporation has a significant contribution to the total heat dissipation, especially at low heat fluxes.