Experimental study of boiling phenomena and heat transfer performances of FC-72 over micro-pin-finned silicon chips

Experiments were conducted to study the effects of micro-pin-fins on boiling phenomena and heat transfer from square simulated silicon chips immersed in a pool of FC-72. Two kinds of micro-pin-fins having fin thickness of 30 m and fin heights of 60 and 200 m, respectively, were fabricated on the silicon chip surface with the dry etching technique. The experiments were conducted at the liquid subcoolings of 3, 25, 35 and 45 K. The effects of dissolved air in FC-72 and chip orientation were also investigated. The boiling curve of the micro-pin-finned chips was characterized by a very small increase in wall superheat with increasing heat flux, and the wall temperatures at the CHF point for all the micro-pin-finned chips were less than the upper limit for the reliable operation of LSI chips (T_w=85°C). Liquid subcooling was very effective in elevating CHF for the micro-pin-finned chips compared to the smooth surface and other treated surfaces. The enhanced boiling heat transfer mechanisms for the micro-pin-finned chips were discussed.     

The role of enhanced coated surface in pool boiling CHF in FC-72

 The purpose of Critical heat flux (CHF) experiments was to determine the role of various types and thickness of enhanced coated surface on a horizontal, vertically oriented ribbon heaters made of Ti and Steel 1010 of different thickness. Saturated pool boiling in FC-72 at atmospheric pressure was used in the experiment. The microstructure and surface topography are important factors in pool boiling CHF. In conditions of highly wetting FC-72 and increased roughness, CHF increased by 6 to 12%. However, CHF increased by 29% with greater topographic unevenness of the surface and lower roughness, which was obtained by etching Steel 1010 in H₂SO₄ acid. CHF also increased when the content of metals and metal oxides particles in the coating were increased. The CHF ratio of enhanced coated surface to a ribbon heater featuring a standard surface finish is up to 2.3. In addition, the asymptotic CHF of these uncoated heaters was considerably exceeded. Received on 17 January 2000     

Experimental study of pool temperature effects on nucleate pool boiling

Nucleate pool boiling experiments with constant wall temperature were performed using pure R113 for subcooled, saturated, and superheated pool conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant wall temperature and to measure the instantaneous heat flow rate accurately with high temporal and spatial resolutions. Images of bubble growth were taken at 5000 frames per second using a high-speed CCD camera synchronized with the heat flow rate measurements. The bubble geometry was obtained from the captured bubble images. The effect of the pool conditions on the bubble growth behavior was analyzed using dimensionless parameters for the initial and thermal growth regions. The effect of the pool conditions on the heat flow rate behavior was also examined. The bubble growth behaviors during subcooled, saturated, and superheated pool boiling were analyzed using a modified Jakob number that we newly defined. Dimensionless time and bubble radius parameters with the modified Jakob number characterized the bubble growth behavior well. These phenomena require further analysis for various pool temperature conditions, and this study will provide good experimental data with precise constant wall temperature boundary condition for such works.     

FC-72 pool boiling from finned surfaces placed in a narrow channel: preliminary results

This paper presents an experimental study dealing with the basic nucleate boiling concerning two finned surfaces placed in a narrow channel. The influence of both the channel width and the orientation of the base surface (horizontal or vertical) are discussed. The experiments were performed in a saturated pool of FC-72 while the channel widths investigated were 2.0 mm and 0.5 mm. The experimental data are compared with those obtained in the case of the unconfined situation of the extended surfaces. Channel width reduction does not affect the heat transferred to the liquid in the case of vertical orientation of the base surface, while it causes a drastic reduction in the heat transfer behavior in the case of a horizontal base surface. For the latter situation, vapor stagnation in the gap was observed after the maximum heat flux had been reached. Received on 13 August 1998     

Nucleate boiling of FC-87/FC-72 zeotropic mixtures on a horizontal copper disc

This paper presents nucleate boiling experimental results, at atmospheric pressure, for heat fluxes q ≤ 40 kW/m², for FC-87/FC-72 binary mixtures in molar fractions of 0/100, 25/75, 50/50, 75/25, 85/15 and 100/0, at saturation temperatures for pure fluids and bubble points for mixtures. The test section was an upward facing copper disc of 12 mm diameter and 1 mm thickness. The experimental heat transfer coefficient was compared with the correlations of Rohsenow (1952), as reported by Rohsenow et al. (Handbook of heat transfer, McGraw-Hill, New York, 1998), Stephan and Abdelsalam (Int J Heat Mass Transfer 23;73–78, 1978) and Cooper (Int Chem Eng Symp Ser 86:785–792, 1984) for pure fluids and the semi-empirical models of Stephan and Körner (Chem Ing Tech Jahrg 7:409–484, 1969), Thome (J Heat Transfer 104:474–478, 1982), Fujita et al. (1996), as reported by Rohsenow et al. (Handbook of heat transfer, McGraw-Hill, New York, 1998), Fujita and Tsutsui (Int J Heat Mass Transfer 37(1):291–302, 1994) and Calus and Leonidopoulos (Int J Heat Mass Transfer 17:249–256, 1973) for mixtures.     

Flow boiling enhancement of FC-72 from microporous surfaces in minichannels

The microporous coatings can remarkably enhance the liquid boiling heat transfer. Therefore, they are promising to be introduced into minichannels in the design of the cooling system of high-power microchips. However, the flow boiling heat transfer characteristics from microporous surfaces in the minichannels have not been extensively studied, and the pertinent knowledge is rather fragmentary. The present research is an experimental investigation on flow boiling of a dielectric fluid FC-72 from microporous coating surfaces in horizontal, rectangular minichannels of 0.49, 0.93 and 1.26 mm hydraulic diameter. Effects of coating structural parameters, such as the particle diameter and coating thickness, were investigated to identify the optimum microporous coating for heat transfer enhancement. All microporous surfaces in this paper were found to significantly enhance FC-72 flow boiling heat transfer in minichannels. With the optimum coating, the heat transfer coefficients could be 7-10 times those of the uncoated surface, and the boiling wall temperature was reduced by about 10 K. The flow boiling phenomena in the present minichannels were distinctly different from those in conventional-sized channels, due to the wall confinement effect on vapor bubbles. The confinement effect was evaluated by taking the contributions of the liquid mass flux and channel size into consideration. It was found that the very strong confinement effect was unfavorable with respect to flow boiling enhancement of the microporous coatings in the minichannels.     

Experimental studies on CHF enhancement in pool boiling with CuO-water nanofluid

Critical heat flux enhancement (CHF) in pool boiling with CuO nanofluids was experimentally studied using a 36 gauge NiCr wire at atmospheric pressure. Experimentation included (1) subjecting the wire surface to multiple heating cycles with constant volume concentration of CuO nanofluid and (2) subjecting the wire surface to a single heating cycle with different volume concentrations of CuO nanofluid. Boiling of nanofluid in both the cases resulted in nanoparticle deposition and subsequent smoothing of the wire surface. To substantiate the nanoparticle deposition and its effect on critical heat flux, investigation was done by studying the surface roughness and SEM images of the wire surface. The experimental results show the evidence of nanoparticle deposition on the wire surface and its effect on CHF enhancement.     

Experimental studies on CHF characteristics of nano-fluids at pool boiling

To investigate the CHF characteristics of nano-fluids, pool boiling experiments of nano-fluids with various concentrations of TiO₂ or Al₂O₃ nanoparticles were carried out using a 0.2 mm diameter cylindrical Ni–Cr wire under atmospheric pressure. The results show that the CHFs of various nano-fluids are significantly enhanced over that of pure water. SEM observation subsequent to the CHF experiment revealed that a nanoparticle coating is generated on the wire surface during pool boiling of nano-fluids. The CHF of pure water was measured on a nanoparticle-coated wire which was produced during the pool boiling experiments of nano-fluids. The CHF of pure water on the nanoparticle-coated wire was similar to that of nano-fluids. This result clearly shows that the main reason for CHF enhancement of nano-fluids is the modification of the heating surface by the nanoparticle deposition. The nanoparticle-coated surface was characterized with various parameters closely related to pool boiling CHF: surface roughness, contact angle, and capillary wicking performance. Finally, CHF enhancement of nano-fluids is discussed using the parameters.     

Two-phase pressure drop and flow visualization of FC-72 in a silicon microchannel heat sink

The rapid development of two-phase microfluidic devices has triggered the demand for a detailed understanding of the flow characteristics inside microchannel heat sinks to advance the cooling process of micro-electronics. The present study focuses on the experimental investigation of pressure drop characteristics and flow visualization of a two-phase flow in a silicon microchannel heat sink. The microchannel heat sink consists of a rectangular silicon chip in which 45 rectangular microchannels were chemically etched with a depth of 276 μm, width of 225 μm, and a length of 16 mm. Experiments are carried out for mass fluxes ranging from 341 to 531 kg/m² s and heat fluxes from 60.4 to 130.6 kW/m² using FC-72 as the working fluid. Bubble growth and flow regimes are observed using high speed visualization. Three major flow regimes are identified: bubbly, slug, and annular. The frictional two-phase pressure drop increases with exit quality for a constant mass flux. An assessment of various pressure drop correlations reported in the literature is conducted for validation. A new general correlation is developed to predict the two-phase pressure drop in microchannel heat sinks for five different refrigerants. The experimental pressure drops for laminar-liquid laminar-vapor and laminar-liquid turbulent-vapor flow conditions are predicted by the new correlation with mean absolute errors of 10.4% and 14.5%, respectively.     

Experimental studies on the effect of tube inclination on nucleate pool boiling

Experiments were conducted to study the effect of tube inclination on nucleate pool boiling heat transfer for different tube diameters and surface roughness values. The results show that as the tube is tilted from the vertical to the horizontal, the temperatures at the top and bottom (with respect to circumference) increase and decrease, respectively. The increase and decrease is such that they almost compensate for each other, resulting in very little variation of the average heat transfer coefficient with tube inclination. The increase in bubble sliding length at the bottom wall and decrease in bubble sliding length at the top wall are thought to be the reasons for this behaviour. Experiments have been conducted with water, ethanol and acetone at atmospheric pressure, to confirm similar effects of inclination irrespective of fluid property.     

Experimental study on bubble departure characteristics in subcooled nucleate pool boiling

The boiling models use departure diameter and frequency in closure relations for the calculation of nucleate boiling heat flux. These parameters are normally derived from empirical correlations which depend heavily on experiments. While these parameters are studied mostly for saturated conditions, there is not sufficient data for the values of departure diameter and frequency in subcooled boiling. In this work, the bubble departure characteristics, i.e. the departure diameters and frequency have been measured using high speed visualization experiments with subcooled demineralized water at atmospheric pressure for nucleate pool boiling conditions. The water pool dimensions were 300 mm × 135 mm × 250 mm with four different heating elements to carry out the parametric studies of bubble departure behavior. The considered parameters were heater surface roughness, heater geometry and heater inclination along with the experimental conditions like degree of subcooling (ΔT_sub = 5−20 K), superheat (ΔT_sat = 1−10 K) and the heat flux. The departure diameters and frequencies were directly measured from the images captured. It was intended to generate the subcooled nucleate pool boiling data under a wide range of conditions which are not present in the literature. The departure diameter was found to increase with the wall superheat, heater size and the inclination angle while the liquid subcooling and surface roughness produced a damping effect on the diameter. The departure frequency was found to increase with the wall superheat and the inclination angle, but decreases with an increase in the heater size. The frequency increases with the degree of subcooling except very close to the saturation, and is unaffected by the surface roughness beyond a certain superheat value.     

Single bubble growth in saturated pool boiling on a constant wall temperature surface

Nucleate pool boiling experiments with constant wall temperatures were performed using R11 and R113 for saturated pool boiling conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain a constant wall temperature condition and to obtain measurements with high temporal and spatial resolution. Accurate heat flow rate data were obtained from microscale heater array by controlling the surface conditions at a high temporal resolution. Images of the bubble growth were captured using a high-speed CCD camera synchronized with the heat flow rate measurements. The geometry of the bubble was obtained from the images. In the asymptotic growth region, the bubble showed a growth rate that was proportional to t^1/5, which was slower than the growth rate proposed in previous analytical analyses. The bubble growth behavior was analyzed using a new dimensionless parameter to permit comparisons with previous results at the same scale. The comparisons showed good agreement in the asymptotic growth region. A non-dimensional correlation for the bubble radius that can predict the bubble growth and the heat flow rate simultaneously, was suggested. The required heat flow rate for the volume change of the observed bubble was estimated to be larger than the instantaneous heat flow rate measured from the wall. Heat, other than the instantaneous heat supplied from the wall, is estimated to be transferred through the interface between bubble and liquid, even with saturated pool conditions. This phenomenon under a saturated pool condition needs to be analyzed and the data from this study can supply the good experimental data with the precise boundary condition (constant wall temperature).     

Experimental investigation of flow and heat transfer for the ethanol-water solution and FC-72 in rectangular microchannels

Rapid development of super scale integration circuit (IC) provides unprecedented challenge to thermal control for aviation electronic equipments. To solve the problem of cooling electronic chips and devices for aircraft avionics, this paper experimentally investigated the characteristics of single-phase forced convection heat transfer and flow resistance in rectangular microchannels with two liquid coolants. One was 30% of ethanol–water solution, the most commonly used coolant in aviation. The other was FC-72, the latest coolant for electronic equipments. Based on the experimental data collected and those available in the open literature, comparisons and analyses were carried out to evaluate the influences of liquid velocity, supercooling temperature, microchannel structures and wall temperature etc. on the heat transfer behaviors. And the correlations of flow resistance and heat transfer characteristics were provided for the ethanol–water solution and FC-72 respectively. The results indicate transition from laminar to turbulent flow occurs at the Reynolds number of 750–1,250 for FC-72, and the behaviors of flow and heat transfer in rectangular microchannels strongly depend on the kind of coolant and geometric configuration of microchannels.     

Visualization of pool boiling on enhanced surfaces

This work reports experiments to visualize nucleate boiling on an enhanced tubular surface having sub-surface tunnels and surface pores. The finned copper tube had 1575 fins/m (40 fins/in.) and 0.8 mm fin height. The fins are covered by a thin foil sheet having 0.23 mm pores at 1.5 mm pore pitch along each interfin region. Data are provided for two foil cover sheets, one copper and the other a transparent plastic. The uniqueness of this work is that the visualization method allowed direct observation of the boiling process in the subsurface tunnels. Use of a high speed camera with 30 × magnification allowed detailed observation of the evaporation process in the tunnels and of the vapor bubbles emerging from the pores. The experiments were conducted for saturated and subcooled boiling in the horizontal and vertical orientations. For the vertical tube, the saturated boiling experiments showed that all of the tunnels were vapor filled except for liquid menisci in the corners. This was also true for the horizontal tube at high heat flux. For the horizontal tube at low heat flux, portions of the tunnel length was liquid filled. A large portion (70–90%) of the region was vapor filled except for liquid menisci in the corners, and the remaining part of the region had oscillating menisci. These experiments provide conclusive proof that the heat transfer mechanism in the subsurface tunnels is evaporation on the menisci in the corners.     

Circumferential temperature distribution during nucleate pool boiling outside smooth and modified horizontal tubes

In the work an approach to avoid a circumferential temperature distribution existing during nucleate pool boiling on a horizontal cylinder within low heat flux densities is presented. The idea of the approach is local heat transfer enhancement by a porous layer application on a part of the heating surface. An experiment on nucleate pool boiling heat transfer from horizontal cylinders to saturated R141b and water under atmospheric pressure is reported. Experiments have been conducted using stainless steel tubes with the outside diameter between 8 mm and 23 mm with the active length of 250 mm. The outside surface of the tubes was smooth or partially coated with a porous metallic layer. In particular, measurements of inside circumferential temperature distribution have been performed.     

Experimental study of the characteristics and mechanism of pool boiling CHF enhancement using nanofluids

The pool boiling critical heat flux (CHF) performance of aqueous nanofluids was investigated using various nanoparticles of TiO₂, Al₂O₃, and SiO₂. The usage of a nanofluid as a working fluid can significantly enhance the pool boiling CHF, which was found to be strongly dependent on the kind of nanoparticle, as well as its concentration. A nanoparticle surface coating was observed on the heating surface after the experiment. The CHF of pure water on the nanoparticle-coated surface was higher than that of nanofluids for all cases. This revealed that the cause of the CHF enhancement using nanofluids is due to the fact that the heater surface is modified by the nanoparticle deposition. The mechanism of CHF enhancement due to the nanoparticle coating was discussed, relating it to surface wettability, surface roughness, and maximum capillary wicking height of the nanoparticle-coated surface.