Issam Mudawar

Experimental and numerical study of pressure drop and heat transfer in a single-phase micro-channel heat sink

The pressure drop and heat transfer characteristics of a single-phase micro-channel heat sink were investigated both experimentally and numerically. The heat sink was fabricated from oxygen-free copper and fitted with a polycarbonate plastic cover plate. The heat sink consisted of an array of rectangular micro-channels 231 lm wide and 713 lm deep. Deionized water was employed as the cooling liquid and two heat flux levels, q 00 ¼ 100 W=cm 2 and q 00 ¼ 200 W=cm 2 , defined relative to the planform area of the heat sink, were tested. The Reynolds number ranged from 139 to 1672 for q 00 ¼ 100 W =cm 2 , and 385 to 1289 for q 00 ¼ 200 W =cm 2 . The three-dimensional heat transfer characteristics of the heat sink were analyzed numerically by solving the conjugate heat transfer problem involving simultaneous determination of the temperature field in both the solid and liquid regions. Also presented and discussed is a detailed description of the local and average heat transfer characteristics of the heat sink. The measured pressure drop and temperature distributions show good agreement with the corresponding numerical predictions. These findings demonstrate that the conventional Navier–Stokes and energy equations can adequately predict the fluid flow and heat transfer characteristics of micro-channel heat sinks. 2002 Elsevier Science Ltd. All rights reserved.

High flux boiling in low flow rate, low pressure drop mini-channel and micro-channel heat sinks

Abstract Due to the need for practical cooling technologies which could dissipate high heat fluxes, an experimental study of pressure drop and CHF in mini-channel ( D = 2.54 mm ) and micro-channel ( D = 510 μm ) heat sinks of 1 cm heated length was performed using R-113. Test conditions included inlet subcooling ranging from 10 to 32°C and a range of low flow rates up to a maximum of 95 ml min−1. The tests yielded CHF values for both heat sinks in excess of 200 W cm−2 with the advantage of both low flow rates and low pressure drops ( ΔP ) as compared to high-flux, single-phase micro-channel heat sinks. Key features of the miniature heat sinks include a lack of inlet subcooling effect on CHF and superheated outlet conditions at the lowest flow rates. A single CHF correlation was developed for both heat sinks. The paper will illustrate the use of the CHF correlation and a generalized model for pressure drop as predictive tools in assessing the merits of different channel sizes in incorporating miniature heat sink technology into high heat flux cooling schemes. Overall, the mini-channels performance proved superior to the micro-channel due to pressure drops less than 0.01 bar for comparable CHF values as well as the reduced likelihood of clogging and the relative ease in fabricating the mini-channel.

Correlation of Sauter mean diameter and critical heat flux for spray cooling of small surfaces

Experiments were performed to understand better nucleate boiling heat transfer and critical heat flux (CHF) for full cone sprays. The effects of spray nozzle, volumetric flux, subcooling and working fluid were investigated. Dense sprays greatly reduced evaporation efficiency, and their boiling curves exhibited an unusually small increase in slope upon transition between the single phase and nucleate boiling regimes. Sauter mean diameter (SMD) data were successfully correlated for fluids with vastly different values of surface tension. This correlation was based upon orifice diameter and the Weber and Reynolds numbers of the orifice flow prior to liquid breakup. Also developed was a new CHF correlation which accurately predicted data for FC-72, FC-87 and water. This correlation shows a strong dependence of CHF on volumetric flux and Sauter mean diameter. It is shown that by combining the correlations for CHF and SMD it is possible to predict accurately CHF for full cone sprays without having to conduct expensive and laborious drop sizing measurements for each individual nozzle.

Analysis of three-dimensional heat transfer in micro-channel heat sinks

In this study, the three-dimensional fluid flow and heat transfer in a rectangular micro-channel heat sink are analyzed numerically using water as the cooling fluid. The heat sink consists of a 1-cm 2 silicon wafer. The micro-channels have a width of 57 lm and a depth of 180 lm, and are separated by a 43 lm wall. A numerical code based on the finite difference method and the SIMPLE algorithm is developed to solve the governing equations. The code is carefully validated by comparing the predictions with analytical solutions and available experimental data. For the microchannel heat sink investigated, it is found that the temperature rise along the flow direction in the solid and fluid regions can be approximated as linear. The highest temperature is encountered at the heated base surface of the heat sink immediately above the channel outlet. The heat flux and Nusselt number have much higher values near the channel inlet and vary around the channel periphery, approaching zero in the corners. Flow Reynolds number affects the length of the flow developing region. For a relatively high Reynolds number of 1400, fully developed flow may not be achieved inside the heat sink. Increasing the thermal conductivity of the solid substrate reduces the temperature at the heated base surface of the heat sink, especially near the channel outlet. Although the classical fin analysis method provides a simplified means to modeling heat transfer in micro-channel heat sinks, some key assumptions introduced in the fin method deviate significantly from the real situation, which may compromise the accuracy of this method. 2002 Elsevier Science Ltd. All rights reserved.

Mapping of impact and heat transfer regimes of water drops impinging on a polished surface

Still and high-speed photographic techniques were used to record the impact behavior of water droplets on a hot aluminum surface. Drop velocity and surface temperature were two important parameters governing both the impact behavior and ensuing heat transfer. Droplet Weber numbers of 20, 60 and 220 identified three major classes of impact behavior, while surface temperatures ranging from 280 to 100°C were used to define heat transfer regimes corresponding to film boiling, transition boiling, nucleate boiling, and film evaporation. Temperatures corresponding to the critical heat flux and the Leidenfrost point showed little sensitivity to both droplet velocity and impact frequency. The photographic results and heat transfer measurements were used to construct droplet impact regime maps which identify the various boiling regimes for each of the three Weber numbers. These maps serve as a new useful foundation for understanding droplet impact behavior as well as future analytical or numerical modeling of droplet and spray heat transfer.

Optimizing and Predicting CHF in Spray Cooling of a Square Surface

Spray cooling of a hot surface was investigated to ascertain the effect of nozzle-to-surface distance on critical heat flux (CHF). Full cone sprays of Fluorinert FC-72 and FC-87 were used to cool a 12.7 X 12.7 mm 2 surface. A theoretical model was constructed that accurately predicts the sprays volumetric flux (liquid volume per unit area per unit time) distribution across the heater surface. Several experimental spray sampling techniques were devised to validate this model. The impact of volumetric flux distribution on CHF was investigated experimentally. By measuring CHFfor the same nozzle flow rate at different nozzle-to-surface distances, it was determined CHF can be maximized when the spray is configured such that the spray impact area just inscribes the square surface of the heater. Using this optimum configuration, CHF data were measured over broad ranges of flow rate and subcooling, resulting in a new correlation for spray cooling of small surfaces.

Contact angle temperature dependence for water droplets on practical aluminum surfaces

Abstract This paper presents an experimental investigation of the temperature dependence of the quasistatic advancing contact angle of water on an aluminum surface polished in accordance with surface preparation techniques commonly employed in boiling heat transfer studies. The surface, speculated to contain aluminum oxide and organic residue left behind from the polishing process, was characterized with scanning electron microscopy, surface contact profilometry, and ellipsometry. By utilizing a pressure vessel to raise the liquid saturation temperature, contact angles were measured with the sessile drop technique for surface temperatures ranging from 25 to 170°C and pressures from 101.3 to 827.4 kPa. Two distinct temperature-dependent regimes were observed. In the lower temperature regime, below 120°C, a relatively constant contact angle of 90° was observed. In the high temperature regime, above 120°C, the contact angle decreased in a fairly linear manner. Empirical correlations were developed to describe this behavior which emulated previous experimental data for nonmetallic surfaces as well as theoretical trends.

Flow boiling heat transfer in two-phase micro-channel heat sinks-II. Annular two-phase flow model

This paper is Part II of a two-part study devoted to measurement and prediction of the saturated flow boiling heat transfer coefficient in water-cooled micro-channel heat sinks. Part I discussed the experimental findings from the study, and identified unique aspects of flow boiling in micro-channels such as abrupt transition to the annular flow regime near the point of zero thermodynamic equilibrium quality, and the decrease in heat transfer coefficient with increasing quality. The operating conditions of water-cooled micro-channels fell outside the recommended range for most prior empirical correlations. In this paper, an annular flow model is developed to predict the saturated flow boiling heat transfer coefficient. Features unique to two-phase micro-channel flow, such as laminar liquid and vapor flow, smooth interface, and strong droplet entrainment and deposition effects, are identified and incorporated into the model. The model correctly captures the unique overall trend of decreasing heat transfer coefficient with increasing vapor quality in the low vapor quality region of micro-channels. Good agreement is achieved between the model predictions and heat transfer coefficient data over broad ranges of flow rate and heat flux. 2003 Elsevier Science Ltd. All rights reserved.

Ultra-high critical heat flux (CHF) for subcooled water flow boiling—I: CHF data and parametric effects for small diameter tubes

Ultra-high critical heat flux (CHF) data, with many values exceeding 100 MW m−2, were obtained using high mass velocity, subcooled water flow through short, small diameter tubes. These tests produced the highest CHF of 276 MW m−2 reported in the literature for a uniformly heated tube which surpassed the prior record of 228 MW m−2. The data include broad ranges of tube diameter (0.406–2.54 mm) , heated length-to-diameter ratio (2.4–34.1) , mass velocity (5000–134 000 kg m−2 s−1) , inlet temperature (18–70°C) , and outlet pressure (2.5–172.4 bars) . The parametric trends of CHF were ascertained relative to all important flow and geometrical parameters. CHF increased with increasing mass velocity, increasing subcooling, decreasing tube diameter, and decreasing heated length-to-diameter ratio. For a constant inlet temperature, CHF increased with increasing pressure for pressures up to 30 bars, remained fairly constant between 30 and 150 bars, and decreased afterwards as the critical pressure was approached. CHF was accompanied by physical burnout of the tube wall near the exit and tube material had little effect on the magnitude of CHF. The pressure drop for most conditions was fairly constant, albeit as high as 153.4 bars, over the entire range of heat fluxes, from the single-phase flow condition corresponding to zero heat flux up to CHF, proving that CHF was triggered even with negligible net vapor production. These high pressure drops indicate special attention should be exercised when employing the high mass velocity flows necessary to attaining ultra-high CHF. These high pressure drops also render the practice of referencing CHF data reSlative to a single measured pressure value very misleading.

Effects of surface roughness on water droplet impact history and heat transfer regimes

Abstract Still and high speed photographic techniques and heat transfer measurements were used to study the impact of water droplets on heated surfaces with different roughness. The study encompassed droplet Weber numbers of 20, 60 and 220 and surface temperatures of 100–280°C, covering droplet stability and heat transfer regimes established previously by the authors for polished surfaces. Three different surface finishes, polished, particle blasted and rough sanded, with respective arithmetic average surface roughness values of 97, 970 and 2960 nm, were applied to the test surfaces. While the temperature corresponding to critical heat flux (CHF) was fairly independent of surface roughness, the Leidenfrost point (LFP) temperature was especially sensitive to surface finish. The parametric effects of Weber number and surface temperature were consolidated into droplet impact regime maps for each of the three surface finishes. Aside from depicting the commonly known boiling curve regimes of film, transition and nucleate boiling, and thin film evaporation, these maps illustrate the complex liquid-solid interactions which occur during the lifetime of the impacting droplet within each of the boiling curve regimes, thus serving as an effective reference for future modeling of droplet heat transfer. Copyright