Christopher R. Yap

National University of Singapore, Tumasik, Singapore

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Publications (35)59.11 Total impact

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    Omer Bugra Kanargi · Matthew Law · Christopher Yap · Poh Seng Lee
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    ABSTRACT: The thermal and hydraulic performances of an air-cooled heat sink channel design have been numerically investigated in 2D computational domains in ANSYS Fluent. The channel expansion ratio (ER) is the key parameter to control the flow restriction, therefore the rate of flow mixing. As the channel ER is increased, Nu number increases with an accompanying increase in the pressure drop penalty. The heat transfer enhancement is attributed to two main phenomena: flow mixing, and thermal boundary layer (TBL) disruption and reinitialization. Thanks to flow mixing, air temperature builds up more uniformly in the direction of flow and TBL disruption and reinitialization decreases the thermal resistance on the channel walls. Despite the increase in the pressure drop penalty with increasing channel ER; it is possible to enhance thermal resistance by 40% with the same pressure drop of a straight channel heat sink.
    Preview · Article · Aug 2015 · Energy Procedia
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    ABSTRACT: In this experimental work, the density, dynamic viscosity and higher heating value of methyl ester based waste cooking palm-biodiesel oil (WMEPB) was investigated under varying temperature and blend ratio condition with No. 2 diesel fuel. The transesterified fatty acid methyl ester of palm vegetable oil collected from local food and beverage shops was used as neat biodiesel. Four different fuel blends (20%, 40%, 60% and 80% by volume mixing with base diesel) were studied along with base No. 2 diesel fuel and pure biodiesel. Tests for dynamic viscosity and density were performed in the temperature range 0–130 °C for each fuel sample whereas the higher heating values were determined at 25 °C room temperature condition. It is found that pure biodiesel has the highest density and dynamic viscosity at a given temperature whereas it exhibits lowest combustion heating value among the six fuels. Moreover, the density for each fuel sample decreases linearly with the increase in temperature. On the other hand, the dynamic viscosity decreases exponentially with the temperature for each fuel sample. In addition, based on the experimental results, regression correlations have been proposed for the density, dynamic viscosity, and higher heating value of the fuels. Subsequently, comprehensive error analyses of these proposed correlations were performed. In particular, the correlation for density and dynamic viscosity were respectively compared with Kay's mixing rule and Grunberg-Nissan mixing rule theory in order to validate their applicability. It is found that density correlations predicted within ±0.3% average error band. And, as high as 72.2% of the dynamic viscosity data were in the range of ±5% average error while the remaining data fell within ±10% error range. And finally, through a comparative study with the available fuel property results of fresh methyl ester palm biodiesel, it is found that available existing correlations derived from fresh palm biodiesel studies can not accurately predict the fuel properties of same waste biodiesel and its blends with diesel.
    Full-text · Article · Aug 2014 · Renewable Energy
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    Tamanna Alam · Poh Seng Lee · Christopher R. Yap
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    ABSTRACT: Understanding the influence of surface characteristics on flow boiling heat transfer behavior in microgap is necessary to enhance the performance of microgap heat sink. The influences of surface roughness on flow boiling heat transfer, pressure drop and instability in microgap heat sink are experimentally investigated. Flow boiling experiments are conducted over silicon microgap heat sink of three different microgap dimensions namely 500 μm, 300 μm and 200 μm. The original silicon surface of surface roughness, Ra = 0.6 μm is modified to Ra = 1.0 μm and 1.6 μm to examine the effect of surface finish. These studies are carried out with the inlet deionized water temperatures 91 °C at two different mass fluxes, G = 390 kg/m2 s and 650 kg/m2 s and imposed effective heat flux, View the MathML source ranging from 0 W/cm2 to 85 W/cm2. High speed flow visualizations are conducted simultaneously along with experiments to explore the bubble behavior in microgap heat sink. The results of this study show that bubble nucleation site density as well as heat transfer coefficient increases with the increase of surface roughness and pressure drop is independent of surface roughness in microgap heat sink. Moreover, rougher surface maintains lower and uniform wall temperature over the heated surface. However, surface roughness has an adverse effect on the inlet pressure instability and inlet pressure fluctuation increases with increasing surface roughness at larger microgap heat sink.
    Full-text · Article · Sep 2013 · International Journal of Heat and Mass Transfer
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    Tamanna Alam · Poh Seng Lee · Christopher R. Yap · Liwen Jin
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    ABSTRACT: Two phase microgap heat sink has a large potential to minimize the drawbacks associated with two phase microchannel heat sink, especially flow instabilities, flow reversal and lateral variation of flow and wall temperature between channels. This new concept of the two-phase microgap heat sink is very promising due to its high heat transfer rate and ease of fabrication. However, comparison of the performance of microgap heat sink (heat transfer, pressure drop and instability characteristics) with some conventional heat sink has not been investigated extensively. In this study, experiments have been conducted to investigate the heat transfer and pressure drop characteristics of deionized water (DI) in microgap heat sink and compare these experimental results with similar data obtained for microchannel heat sink. These studies are carried out with the inlet DI water temperatures 86 °C at different mass fluxes ranging from 400 to 1000 kg/m2 s, for effective heat flux 0–85 W/cm2. High speed flow visualizations are conducted simultaneously along with experiments to illustrate the bubble characteristics in the microchannel and microgap heat sink. Experimental result shows that microgap heat sink performs better at high heat flux and low mass flux due to confined slug and annular boiling dominance and consequent delay of dryout phase. So, this microgap technology is promising and an effective method to dissipate very high heat fluxes in compact space with a smaller rate of coolant flow. Moreover, pressure drop is higher in microchannel than microgap heat sink at all the heat fluxes. In addition, encouraging results have been obtained using microgap as it can potentially mitigate local hotspot, reduce flow instabilities, flow reversal and maintain uniform wall temperatures over the heated surface.
    Full-text · Article · Mar 2013 · International Journal of Heat and Mass Transfer
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    M. Ebna Alam Fahd · Yang Wenming · P. S. Lee · S. K. Chou · Christopher R. Yap
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    ABSTRACT: A detailed experimental study has been conducted to evaluate the effect of 10% water emulsion diesel (ED10) on engine performance and emission, and comparison is made against base diesel fuel. The experiments were performed in a four cylinder 2.5 L DI turbocharged Toyota diesel engine at four different engine loading conditions (25%, 50%, 75% and 100% load). During experiments, the engine speed was varied from 800 rpm to 3600 rpm in steps of 400 rpm for each load condition. Results of in-cylinder pressure traces, heat release rate, engine power output, brake thermal efficiency and brake specific fuel consumption is presented as engine performance parameters while measurement of exhaust gas temperature, nitric oxide (NO) and carbon mono-oxide (CO) output is reported as emission parameters. It is noted that ED10 has the ability to produce comparable in-cylinder pressure and heat release rate like base diesel fuel. It is also found that ED10 produces slightly less engine power output with higher brake specific fuel consumption (BSFC). In addition, lower exhaust gas temperature and lower NO emission is experienced at all load and engine speed condition for ED10 as compared to diesel fuel. Although diesel engines are not prone to higher CO emission at medium to high engine load, it is found that ED10 suffers from higher CO emission at low load and low engine speed condition. However, at higher engine speed for a particular load, the CO emission reduces significantly. The comprehensive analysis of the experimental results suggests that ED10 has the potential to be considered as a competitive renewable and greener fuel for diesel engine applications. (c) 2012 Elsevier Ltd. All rights reserved.
    Full-text · Article · Feb 2013 · Applied Energy
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    Tamanna Alam · Poh Seng Lee · Christopher R. Yap · Liwen Jin · K. Balasubramanian
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    ABSTRACT: The rapid increase of heat flux in high performance electronic devices has necessitated the development of high capacity thermal management techniques that can support extremely high heat transfer rates. Flow boiling in microgap is very promising for this purpose due to its high heat transfer rate and ease of fabrication. However, the effects of microgap dimension on heat transfer and pressure drop characteristics along with flow visualization have not been investigated extensively. This paper focuses on flow boiling experiments of deionized water in silicon microgap heat sink for ten different microgap dimensions from a range of 80 μm–1000 μm to determine the most effective and efficient range of microgap dimensions based on heat transfer and pressure drop performance. High speed flow visualization is conducted simultaneously along with experiments to illustrate the bubble characteristics in the boiling flow in microgap. The results of this study show that confinement in flow boiling occurs for microgap sizes 500 μm and below and confined slug/annular flow is the main dominant regime whereas physical confinement does not occur for microgap sizes 700 μm and above and bubbly flow is the dominant flow regime. The microgap is ineffective below 100 μm as partial dryout strikes very early and the wall temperature is much higher for a fixed heat flux as microgap size increases above 500 μm. In addition, results show that pressure drop and pressure fluctuation decrease with the increases of gap size whereas wall temperature and wall temperature fluctuation increase with the increases of gap size. A strong dependence of heat transfer coefficient on microgap sizes is observed for microgap sizes 500 μm and below. However, the heat transfer coefficient is independent of microgap size for microgap sizes 700 μm and above.
    Full-text · Article · Dec 2012 · International Journal of Heat and Mass Transfer
  • T. Alam · Poh Seng Lee · C.R. Yap
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    ABSTRACT: Flow boiling instabilities induce mechanical vibration in the system and deteriorate the heat transfer performances, for example- premature dryout, critical heat flux limitation etc. The two phase microgap heat sink has novel potential to mitigate these undesirable flow boiling instabilities and flow reversal issues inherent with two phase microchannel heat sink. This work is an experimental study of boiling instabilities in microgap heat sink for different microgap depths ranging from 80μm-1000μm, mass fluxes from 390kg/m2s-900kg/m2s, heat fluxes up to 85W/cm2 and different microgap surface roughnesses, Ra=0.6-1μm. A series of systematic experiments have been carried out to investigate the inlet pressure and wall temperature oscillations during two phase flow boiling condition under uniform heating, with deionized water as a cooling liquid. Experimental result shows that pressure oscillation increases with the decreasing microgap depth. Temperature oscillation is observed lower for smaller gap than larger gap up to a certain heat flux condition before the dryout phase. In addition, inlet pressure instabilities increase with increasing heat flux and decreasing mass flux. Moreover, surface roughness has an adverse effect on the inlet pressure instability at larger depth microgap heat sink and inlet pressure fluctuation increases with increasing surface roughness.
    No preview · Conference Paper · Dec 2012
  • Tamanna Alam · Poh Seng Lee · Christopher R. Yap · Liwen Jin
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    ABSTRACT: Flow boiling in microgap heat sink is very attractive for high-performance electronics cooling due to its high heat transfer rate and easy fabrication process. In absence of thermal interface material between the active electronic component and a microgap cold plate, significant reduction in interface thermal resistance and enhancement in heat transfer rate can be achieved. In earlier studies by these authors, encouraging results have been obtained using microgap heat sink as it can potentially mitigate flow instabilities, flow reversal and maintain uniform wall temperatures over the heated surface. So, more work should be carried out to advance the fundamental understanding of the two-phase flow heat transfer associated with microgap heat sink and the underlying mechanisms. In this study, local flow boiling phenomena in different microgap sizes have been investigated experimentally. Experiments are performed in silicon based microgap heat sink having microgap depth ranging from 80 μm to 500 μm, using deionized water with 10 °C subcooled inlet temperature. The effects of mass flux and heat flux on heat transfer coefficient and pressure drop characteristics are examined by using different mass fluxes ranging from 400 kg/m2s to 1000 kg/m2s and effective heat flux varying from 0 to 100 W/cm2. Apart from these experimental investigations, simultaneous high speed visualizations are conducted to observe and explore the mechanism of flow boiling in microgap. Confined slug and annular boiling are observed as the two main heat transfer mechanisms in microgap. Moreover, experimental results show that flow boiling heat transfer coefficients are dependent on gap size, and the lower the gap size, higher the heat transfer coefficient.
    No preview · Conference Paper · Nov 2012
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    Tamanna Alam · Poh Seng Lee · Christopher R. Yap · Liwen Jin
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    ABSTRACT: Two-phase microgap channel cooling concept has been recently proposed for cooling the heat sources directly in application of electronic devices thermal management. This concept is relatively new and more research should be carried out systematically to investigate the size effects of microgap channel on heat transfer and pressure drop mechanisms. In this study, local flow boiling phenomenon in different microgap sizes has been investigated experimentally. Experiments are performed in silicon based microgap heat sink having microgap of depth 190 μm, 285 μm and 381 μm, using deionized water with inlet temperature of 86 °C. The effects of mass flux and heat flux on heat transfer coefficient and pressure drop characteristics are examined by using three different mass fluxes 420 kg/m2 s, 690 kg/m2 s and 970 kg/m2 s and effective heat flux varying from 0 to 110 W/cm2. An array of integrated micro-temperature sensors are used in this study to obtain the local temperatures and subsequently local heat transfer coefficients are determined. Apart from these experimental investigations, simultaneous high speed visualizations are conducted to observe and explore the mechanism of flow boiling in microgap channel. The results of this study show that flow boiling heat transfer coefficient is dependent on gap size, and the lower the gap size, higher the heat transfer coefficient. Moreover, it has been observed that confined slug and annular boiling are the dominant heat transfer mechanisms in microgap channels after the onset of nucleate boiling. Hence, local heat transfer coefficient increases significantly because of thin film evaporation during confined boiling at high heat flux. This study also evaluates the effectiveness of microgap cooling technology, to eliminate temperature gradient and hotspots.
    Full-text · Article · Jun 2012 · International Journal of Multiphase Flow
  • Tamanna Alam · Poh Seng Lee · Christopher R. Yap · Liwen Jin
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    ABSTRACT: Hotspots can be generated by non-uniform heat flux condition over the heated surface due to higher packaging densities and greater power consumption of high-performance computing technology in military systems designs. Because of this hotspot within a given chip, local heat generation rate exceed the average value on the chip and increase the peak temperature for a given total power generation which degrades the reliability and performance of equipments. Two phase microgap cooling technology is promising to minimization of temperature gradient and reduction of maximum temperature over the heated surface of the device because of unique boiling mechanism in microgap: confined flow and thin film evaporation. The present study aims to experimentally investigate the applicability of microgap cooling technology for minimizining temperature gradient and mitigating hotspots from the heated surface of electronic device. Experiments are performed in silicon based microgap heat sink having a range of gap dimension from 200 µm – 400 µm. Encouraging results have been obtained using microgap channel cooler for hotspots mitigation as it maintain uniform and low wall temperature over the heated surface.
    No preview · Article · Dec 2011
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    ABSTRACT: The paper presents an adsorption cycle for cooling using the dielectric heating method for the regeneration process. Conventional adsorption (AD) chillers employs thermally driven processes for desorption and adsorption where the thermal resistances are high, resulting in a relatively low chiller COP. In this paper, dielectric heating is used to irradiate where the microwave power vibrates the dipole structure of the molecules. Owing to the direct method of energy delivery, the heating process is thus efficient, contributing to an increase in the chiller performance and the COPs. We present the modeling and simulations of the adsorption-desorption cycle in an AD chiller, demonstrating a significant improvement in chiller performance by as much as three folds.
    No preview · Article · Nov 2011 · Modern Physics Letters B
  • Mark Aaron Chan · Christopher R. Yap · Kim Choon Ng
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    ABSTRACT: This study presents a generalized confined boiling correlation applicable for various working fluids and operating conditions. A dimensionless parameter, Bond number, has been incorporated into the correlation to include the effects of confinement in the ebullition process of boiling. The proposed correlation is compared with an existing correlation based on their capability in predicting confined boiling data from the literature. A phenomenon of heat transfer coefficient stagnation is found for boiling in narrow spaces despite an increase in heat flux. Results show that the proposed correlation entails an excellent agreement with experimental data, and the predictions have a reasonably low mean absolute error of 17.3% for the entire database.
    No preview · Article · Jan 2011 · Journal of Heat Transfer
  • Kim Choon Ng · Christopher R. Yap · Mark Aaron Chan
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    ABSTRACT: This research paper presents a study of boiling heat transfer from longitudinal rectangular and square pin finned surfaces immersed in saturated water at low vapor pressures of 2 and 9 kPa. Conventional boiling analysis, which is based on the nominal surface area of the heater, was compared with a boiling analysis that considers the total "wetted" surface area.
    No preview · Article · May 2010 · Modern Physics Letters B
  • Mark Aaron Chan · Christopher R. Yap · Kim Choon Ng
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    ABSTRACT: This research paper presents a study of boiling heat transfer from longitudinal rectangular-finned surfaces immersed in saturated water at low vapor pressures. Finned surfaces with assorted fin spacing, fin thicknesses, and fin heights on a copper based surface have been investigated. All the finned surfaces were found to increase both boiling heat transfer coefficients and critical heat fluxes. An optimal fin thickness was found for a configuration, and heat transfer coefficients have been obtained at the pressures. Factors affecting the boiling characteristics have been identified and the optimal enhancement requires a balance of the active nucleation sites, bubble flow resistance, natural convection, thin film evaporation, liquid superheating, heat transfer area, bubble coalescence, and liquid reflux resistance. High speed visualization of vapor plug and vapor film generation on the boiling surfaces has revealed significant insights into the boiling mechanisms at low saturation pressures.
    No preview · Article · Mar 2010 · Journal of Heat Transfer
  • Mark Aaron Chan · Christopher R. Yap · Kim Choon Ng
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    ABSTRACT: This paper describes the modeling, design, and testing of a high flux and yet compact two-phase CPU cooler, with excellent attributes of low thermal resistance that are derived from the intrinsic design features of phase change phenomena and minimal vapor pressure drop of the device. For the same footprint of a conventional cooler, the prototype rejects more than twice the capacity of CPUs of today. The unique design minimizes its overall size and yet provides adequate area for forced convection cooling. Testing was conducted over an assorted heat loads and air flow rates flowing through the fins, achieving a best performance of 0.206 K/W of device thermal resistance at a rating of 203 W under an air flow rate of 0.98 m3/min. The prototype device is orientation free where a 90° tilt could perform at the same rating conditions.
    No preview · Article · Jul 2009 · International Journal of Heat and Mass Transfer
  • J.M. He · K.C. Ng · C. Yap · B.B. Saha
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    ABSTRACT: In this paper, the adsorption kinetics of gasoline vapor on pitch-based activated carbon is experimentally investigated. The main objective is to explore the effect of initial bed pressure on the adsorption rate. The experiments have been conducted by using a thermogravimetric analyzer (TGA) unit, which has a precision of ± 0.1 μg in measuring the adsorption uptake, at adsorption temperatures of 30 °C and 35 °C which are useful for gasoline emission control. Pressure is found to influence the adsorption rate and uptake. The adsorption rate constants or effective mass transfer coefficients are correlated with the pressure differences between the gasoline vapor and the adsorption chamber. This study suggests that the effect of the initial pressure of the adsorption chamber should be taken into account in calculating the adsorption kinetics for the simulation of a practical adsorption process.
    No preview · Article · May 2009 · Journal of Chemical & Engineering Data
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    ABSTRACT: Adsorption equilibrium uptake of ethanol onto a highly porous activated carbon based adsorbent, namely Maxsorb III, has been experimentally investigated using a thermo-gravimetric analyzer (TGA) unit over adsorption temperatures ranging from 20 to 60 °C. The Dubinin–Astakhov (D–A) equation has been used to correlate the experimental data. Isosteric heat of adsorption is also estimated by using the Clausius–Clapeyron equation. Employing a thermodynamically equilibrium model, the performance of the ideal adsorption cooling cycle has also been studied and compared to that of activated carbon fiber (ACF)–ethanol pair. Experimental results show that Maxsorb III can adsorb up to 1.2 kg of ethanol per kilogram of adsorbent. Theoretical calculations show that, the Maxsorb III–ethanol adsorption cycle can achieve a specific cooling effect of about 420 kJ kg−1 at an evaporator temperature of 7 °C along with a heat source of temperature 80 °C and thus the pair is recommended for solar cooling applications.
    No preview · Article · Dec 2008 · International Journal of Refrigeration
  • Kim Choon Ng · Christopher R. Yap · Mark Aaron Chan
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    ABSTRACT: Is there a universal method by which engineers and scientists could differentiate the superior design and performance of central processing unit (CPU) coolers from one another? If there is, how would one derive or generate such parameters that span all types of CPU coolers used to-date? An ideal CPU cooling device comprises the following three key characteristics, namely, (i) high capacity for heat removal within the specified thermal design limit of CPUs, (ii) low active power (electricity) consumption, and (iii) compact design requiring low maintenance. The insatiable needs for fast clocking speeds of CPUs through the large scale integration of integrated circuits within a wafer has resulted in system designs with increasingly high heat generation within the wafer's footprint. The International Technology Roadmap for Semiconductors has projected that CPUs would generate up to 200 Watts in the near future. Considering these various aspects and the key role of CPU coolers, this paper is proposing a Figure of Merit and a performance chart to aide in comparing their design and performance.
    No preview · Article · Jul 2008 · Heat Transfer Engineering
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    ABSTRACT: The adsorption isotherms of n-butane on pitch based activated carbon (type Maxsorb III) at temperatures ranging from 298 to 328 K and at different equilibrium pressures between 20 and 300 kPa have been experimentally measured by a volumetric technique. The porous properties such as, the density, Brunauer–Emmett–Teller (BET) surface area, pore size, pore volume along with pore size distribution (PSD) of Maxsorb III have been determined. The Dubinin–Astakhov (DA) adsorption isotherm model describes all of the isotherm experimental data within the acceptable error ranges. The present isotherm data are compared with other published data of activated carbon (AC)/n-butane and showed the superiority of the present findings in terms of uptake capacity. The isosteric heat of adsorption (ΔHads) of n-butane on Maxsorb III is calculated for different loading. Using the adsorption isotherms and ΔHads, the thermodynamic property maps as a function of pressure, temperature and adsorbate amount are also presented.
    No preview · Article · Apr 2008 · International Journal of Heat and Mass Transfer
  • Mark Aaron Chan · Christopher R. Yap · Kim Choon Ng
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    ABSTRACT: CPUs with high clock rates can dramatically increase heat dissipation within their encapsulation due to internal Joule heat from the transistors. The conventionally used air cooling systems for CPUs, such as the aluminum or copper extruded heat sink types, have severe heat transfer “bottlenecks” due to high thermal resistances and they easily reach their thermal design limits (TDL). Alternative cooling devices such as heat pipes and liquid cooling tends to have externally attached radiator/condenser and/or pump and such designs are cumbersome. This paper describes the modeling, design, and testing of a compact (about the size of the Intel stock cooler, diameter: 96mm, height: 50mm), fully integrated, orientation-free, evaporator-condenser device for CPU cooling, with excellent attributes of low thermal resistance from phase change phenomena and minimal vapor pressure drop. The prototype fabricated is designed to reject 200 W (twice the capacity of conventional heat sinks). It is made of copper and uses distilled water as the working fluid. The working fluid boils inside a porous structure clad evaporator and is transported radially to nearby air-cooled condenser sections; this unique arrangement minimizes space while providing adequate area for air convection. Testing was done by subjecting it to varying heat loads and air flow rates. A best performance of 0.206 K/W of the device’s thermal resistance was achieved at a fan air flow rate of 34.5 CFM under 203 W of cooling load, and moreover, these results are in good agreement with the simulation. Further improvement of the current design could yield significantly better performance as the device has yet to reach its full potential, especially with regard to the design of its air-cooled curvilinear fins and boiling enhancement.
    No preview · Conference Paper · Jan 2008