Pool boiling experiments have been conducted with a self-rewetting fluid consisting of an aqueous butanol solution to study the boiling heat transfer enhancement at pressures of 1 ∼ 4 bars. Although self-rewetting fluids have been used to enhance the performance of heat pipes, boiling heat transfer characteristics are yet to be fully understood especially at pressures above atmospheric. Pool boiling experiments with aqueous butanol solutions were performed using an electrically heated platinum wire to obtain pool boiling heat transfer data up to the Critical Heat Flux (CHF). Aqueous butanol solutions with butanol concentrations 2–7% showed enhanced heat transfer coefficients and CHF data at various pressure levels. In comparison to water, aqueous butanol solutions showed 20–270% higher values of CHF at pressures up to 4 bars. The bubble sizes were also observed to be significantly smaller in self-rewetting fluids compared to those in water at the same pressure. This observation was consistent even at higher pressures. However, for the highest butanol concentration tested (7%), the CHF enhancement was diminished at higher pressures.

We focus on condensation and collapse processes of vapor bubble(s) in a subcooled pool. We generate the vapor in the vapor generator and inject it/them to form vapor bubble(s) at a designated temperature into the liquid at a designated degree of subcooling. In order to evaluate the effect of induced flow around the condensing/collapsing vapor bubble, two different boundary conditions are employed; that is, the vapor is injected through the orifice and the tube. We also focus on interaction between/among the condensing/collapsing vapor bubbles laterally injected to the pool. Through this system we try to simulate an interaction between the vapor bubble and the subcooled bulk in a complex boiling phenomenon, especially that known as MEB (microbubble emission boiling) in which a higher heat flux than critical heat flux (CHF) accompanying with emission of micrometer-scale bubbles from the heated surface against the gravity is realized under a rather high subcooled condition.

In this experimental study, we investigate the enhancement of heat transfer in ammonia on a new plate evaporator whose surface is configured with microgrooves. The microgrooves have a depth of 30 μm and a width of 200 μm. The local boiling heat transfer coefficients were measured on the evaporator. To compare the heat transfer characteristics of the evaporator, the local boiling heat transfer coefficient on a flat surface and on two microgrooved surfaces—one vertical and one horizontal to the direction of the ammonia flow—were measured at different ranges of mass flux (2–7.5 kg/m 2 s), heat flux (10–20 kW/m 2 ), and saturation pressure (0.7–0.9 MPa). The results show that the local boiling heat transfer coefficient of the horizontal and vertical microgrooved surfaces was larger than that of a flat surface. In particular, the horizontal microgrooved surface had the best heat transfer coefficient.

The lower limit for the occurrence of homogeneous nucleation boiling explosion during water heating at atmospheric pressure has been determined by applying a new theoretical model proposed by the authors. Two different cases of liquid heating have been considered for the study of homogeneous nucleation boiling explosion. In one case, the liquid on the surface is linearly heated at a rate of 10 K/s to 10 9 K/s. In another case, the liquid suddenly contacts with a high temperature surface such as in case of quenching with jet impingement or droplet. For the linear boundary heating case, the liquid temperature limit at which homogeneous boiling explosion occurs without any cavity or surface effect, essentially corresponds to a value of 302 °C even though the surface is heated very slowly. On the other hand, during water contact with hot surfaces, the occurrence of the homogeneous boiling explosion within a characteristic time period of 1 millisecond is obtained at a maximum liquid temperature of 303 °C for a limiting steady state boundary temperature of about 304 °C. From the definition of the steady-state interface boundary temperature of two 1-D semi-infinite body contact, the lower limiting surface temperatures for the occurrence of the homogeneous nucleation boiling explosion have been determined for water contact with various solid surfaces with different liquid initial temperatures ranging from 0 °C to 100 °C. The effects of the parametric variation in the boundary heating conditions on various characteristics of the homogeneous boiling explosion such as liquid temperature and time of boiling explosion, heat-flux across the liquid-vapor interface at the boiling explosion etc. are also determined and discussed in context with other results available in the literature.

The paper deals with the characteristics of boiling heat transfer phenomena on the metal surfaces where gravitational acceleration between 0g and 1g acts. To conduct the experiment in the field where the gravitational acceleration between 1g and 0g acted accurately, we produced the Atwood machine that was able to obtain the fixed gravitational acceleration field known by physics well. The metallic materials used by the experiment were brass, stainless steel, aluminum, copper and these materials were processed to 10mm in the diameter, and we put these samples in liquid nitrogen and experimented on the boiling phenomenon. As a result, it has been understood that there is the feature shown next in boiling heat transfer phenomena on the metal surface in gravitational acceleration field between 0g and 1g. (1) When brass, copper, stainless steel, and aluminum of the sample were put in the liquid nitrogen, the temperature differentiation coefficient on the sample surface showed the tendency to decrease in proportion to gravitational acceleration changed from 1g into 0g. (2) In boiling heat flux curve in these metals (brass, stainless steel, aluminum and copper), it was clarified for gravitational acceleration 1g to indicate maximum heat flux value q max .

A mechanistic heat transfer correlation is proposed to estimate heat transfer coefficient for non-boiling two-phase flow in horizontal, slightly inclined, and vertical pipes using the analogy between friction factor and heat transfer. Local heat transfer coefficients, pressure drops and flow parameters were measured for air-water flow in a 27.9 mm stainless steel pipe. The heat transfer and pressure drop data were collected by carefully coordinating the gas and liquid superficial Reynolds numbers. The proposed mechanistic correlation is validated by using experimentally measured heat transfer data. Evaluation of the mechanistic correlation with the measured heat transfer data indicated that the analogy between friction factor and heat transfer can be used with reasonable accuracy for heat transfer predictions in non-boiling two-phase pipe flow. Comparison with experimental results showed that the bulk of the data points were predicted within ±30% by the mechanistic model.

An experimental study of mini-jet impingement boiling is presented for saturated and subcooled conditions. Unique to this study is the documentation of boiling curves of submerged water jet impingement under sub-atmospheric conditions. Data are reported at a single sub-atmospheric pool pressure of 0.176 bar and for a fixed nozzle-to-surface distance of six jet diameters. A mini nozzle is used in the present study with an internal diameter of 1.16 mm. Jet impingement boiling at Reynolds numbers in the range of 0 to 6,800 are characterized and contrasted for both saturated and subcooled conditions. Enhancements in critical heat flux with increasing Re are observed for both saturated and subcooled conditions, with the subcooled condition of 17 °C showing approximately 2.3 times the critical heat flux as that observed for saturated conditions. Critical heat flux for subcooled jet impingement boiling is well predicted from the saturated critical heat flux data by a modified subcooled pool boiling CHF correlation presented by Inoue et al. [1]. The effect of surface finish on pool boiling is also reported.

At high superheat bubble growth is rapid and the heat transfer is dominated by radial convection. This has been found, in the case of a droplet boiling within another liquid and in the case of a bubble growing on a heated wall, leading to similar bubble growth curves. Based on experiments conducted for the first case, an empirical model is developed for the prediction of bubble growth within the radial convection dominated regime (the RCD model), occurring only at high superheat (0.26<Ste<0.41). This model shows a dependence of R∼t 1/3 equivalent to Nusselt number decreasing over time (Nu∼t 1/3 ) as opposed to R∼t 1/2 appearing in most other models, leading to a highly unlikely constant Nusselt number. The new model is shown to give accurate predictions for the first case and for the second case at medium-high superheat (0.19<Ste<0.30, experimental data taken from literature). A comparison of the RCD model to other models, shows a more consistent and accurate prediction. However, in the second case (nucleate boiling) the RCD model requires the foreknowledge of the departure diameter, for which a reliable model still is lacking.

The heat transfer and fluid flow characteristics of non-boiling two-phase flow in microchannels were experimentally investigated. The effects of channel diameter (140, 222, 334, and 506 μm) on the Nusselt number were considered. Air and water were used as the working fluids. Results were presented for the Nusselt number over a wide range of gas superficial velocity (1.24–40.1 m/s), liquid superficial velocity (0.57–2.13 m/s), and wall heat flux (0.34–0.95 MW/m 2 ). The results showed that the Nusselt number increased with increasing gas flow rate for the 506 μm and 334 μm channels, while the Nusselt number decreased with increasing gas flow for the 222 μm and 140 μm channels. Based on these experimental results, a transition channel diameter of about 235 μm to 260 μm, which distinguishes microchannels from minichannels, was suggested. By observing two-phase flow patterns within the microchannels, viscosity and surface tension were identified as the key factors that caused the heat transfer characteristics to change. In addition, new correlations for the forced convection Nusselt number were developed.

A framework for scaling pool boiling heat flux is developed using data from various heater sizes over a range of gravity levels. Boiling is buoyancy dominated for large heaters and/or high gravity conditions and the heat flux is heater size independent. The power law coefficient for gravity is a function of wall temperature. As the heater size or gravity level is reduced, a sharp transition in the heat flux is observed at a threshold value of L h /L c = 2.1. Below this threshold value, boiling is surface tension dominated and the dependence on gravity is smaller. The gravity scaling parameter for the heat flux in the buoyancy dominated boiling regime developed in the previous work is updated to account for subcooling effect. Based on this scaling parameter and the transition criteria, a methodology for predicting heat flux in the surface tension dominated boiling regime, typically observed under low-gravity conditions, is developed. Given the heat flux at a reference gravity level and heater size, the current framework allows the prediction of heat flux at any other gravity level and/or heater size under similar experimental conditions. The prediction is validated using data at over a range of subcoolings (7°C ≤ ΔT sub ≤ 32.6°C), heater sizes (2.1 mm ≤ L h ≤ 7 mm), and dissolved gas concentrations (3 ppm ≤ c g ≤ 3500 ppm). The prediction errors are significantly smaller than those from correlations currently available in the literature.

The critical heat flux values of copper substrates were increased from 87 to 125 W/cm 2 by using a simple chemical process resulting in growth of micro and nano-scale copper structures on the surface. Pre- and post-test surface analysis revealed that the morphology of the micro and nano-scale features of these copper structures changed during the boiling process accompanied by a change in oxide layer composition. Boiling performance of the micro and nano-structured samples was repeatable when testing at lower heat fluxes.

This paper presents results of an experimental investigation carried out to determine the effects of surface material on nucleate pool boiling heat transfer of refrigerant R113. Experiments were performed on horizontal circular plates of brass, copper and aluminum. The heat transfer coefficient was evaluated by measuring wall superheat and effective heat flux removed by boiling. The experiments were carried out in the heat flux range of 8 to 200kW/m 2 . The obtained results have shown significant effect of surface material, with copper providing the highest heat transfer coefficient among the samples, and aluminum the least. There was negligible difference at low heat fluxes, but copper showed 23% better performance at high heat fluxes than aluminum and 18% better than brass.

Boiling water in small channels that are formed along turbine blades has been examined since the 1970s as a means to dissipating large amounts of heat. Later, similar geometries could be found in cooling systems for computers, fusion reactors, rocket nozzles, avionics, hybrid vehicle power electronics, and space systems. This paper addresses (a) the implementation of two-phase micro-channel heat sinks in these applications, (b) the fluid physics and limitations of boiling in small passages, and effective tools for predicting the thermal performance of heat sinks, and (c) means to enhance this performance. It is shown that despite many hundreds of publications attempting to predict the performance of two-phase micro-channel heat sinks, there are only a handful of predictive tools that can tackle broad ranges of geometrical and operating parameters or different fluids. Development of these tools is complicated by a lack of reliable databases and the drastic differences in boiling behavior of different fluids in small passages. For example, flow boiling of certain fluids in very small diameter channels may be no different than in macro-channels. Conversely, other fluids may exhibit considerable ‘confinement’ even in seemingly large diameter channels. It is shown that cutting-edge heat transfer enhancement techniques, such as the use of nano-fluids and carbon nanotube coatings, with proven merits to single-phase macro systems, may not offer similar advantages to microchannel heat sinks. Better performance may be achieved by careful optimization of the heat sink’s geometrical parameters and by adapting a new class of hybrid cooling schemes that combine the benefits of micro-channel flow with those of jet impingement.

This study focuses on the clarification of the heat transfer characteristics of the subcooled pool boiling, the discussion on its mechanism, and finally the establishment of a boiling and condensation model for numerical simulation on the subcooled pool boiling phenomena. In this paper, the boiling and condensation model is improved by introducing the following models based on the quasi-thermal equilibrium hypothesis; (1) a modified phase-change model which consisted of the enthalpy method for the water-vapor system, (2) a relaxation time derived by considering unsteady heat conduction. Resulting from the numerical simulations on the subcooled pool boiling based on the MARS (Multi-interface Advection and Reconstruction Solver) with improved boiling and condensation model, the numerical results regarding the bubble growth process of the subcooled pool boiling show in good agreement with the experimental observation results and the existing analytical equations.

Pool boiling experiments were conducted to investigate the saturation boiling of PF-5060 dielectric liquid on micro porous copper surface. The micro porous surface is deposited on a copper coated silicon wafer diced to a size of 40 mm × 68 mm. Reference experiments were performed using a bare silicon wafer of the same size. Experiments are also performed using deionized water that was degassed prior to the experiment. The experimental results show that there is ∼48% enhancement of heat flux in nucleate boiling regime on the micro porous copper surface, compared to that on a bare surface for pool boiling of PF-5060. The measurement uncertainty for heat flux in these experiments is estimated to be ∼15%. The enhanced surface area provided by the micro porous copper surface as well as the reduction in the magnitude of the Taylor instability wavelength on a copper surface, increase in the nucleation site density on the porous surface, capillary replenishment of the dry out regions and the increase in transient heat transfer from the porous surface — are postulated to be the enhancement mechanisms for the observed augmentation in heat flux values.

We investigated the critical heat flux (CHF) for flow boiling of water in a vertical annulus. The coaxial annulus has a diameter ratio of 1.37 and the inner zircaloy tube is heated directly over a length of 325 mm. CHF can occur prematurely due to flow instabilities. Therefore, we analyzed the flow stability at different heat input conditions using two types of pumps, a rotary and a gear type pump. The unstable CHF occurred at 61% and 90% of the stable value for the rotary and the gear type pump, respectively. Consequently, the following CHF experiments were conducted at stable flow conditions. The outlet pressure was constant at 120 kPa, the mass flux varied from 250 to 1000 kg/(m 2 s) and the inlet subcooling was at 102, 167, and 250 kJ/kg. The CHF results increase with mass flux from 0.67 to 2.62 MW/m 2 and show similar trends compared to literature data. However, the experimental data for flow boiling in annuli at low pressure are limited. Additionally, we measured the dynamic contact angle between the zircaloy tube surface and water using the Wilhelmy method.

In this work, we build up a model applying our analogy strategy to investigate the performance of the Bi-porous wicks which are intended to eventually be used as the substrate in the TGP (Thermal Ground Plane) devices which function like thin heat pipes. In order to more closely simulate the operating conditions in a TGP, the vapor space above the wick is restricted, and consequently inserting vapor grooves in the wick is required to maintain the vapor removal. Further, adding a mono-porous layer at the bottom aids in delaying dry out by enhancing the liquid supply. [1] Here, we present our effort improving an electric analogue model to simulate the bi-porous wick described above. The model is based on the similarity between the differential equations governing the two systems. The analog model is initially improved for a disk-shaped bi-porous wick with a restrictor on top and grooves introduced in the bi-porous region. Moreover, the analogue technique is employed as a tool to investigate the performance of the wick with the vapor restrictor mounted thoroughly on top, to compare the experimental data achieved in a boiling chamber versus TGP device, and to estimate the required data for the bi-porous evaporator calculations.

The cooling capacity of two-phase transport in microchannels is limited by the occurrence of critical heat flux (CHF). Due to the nature of the phenomenon, it is challenging to obtain reliable CHF data without causing damage to the device under test. In this work, the critical heat fluxes for flow boiling of FC-77 in a silicon thermal test die containing 60 parallel microchannels were measured at five total flow rates through the microchannels in the range of 20–80 ml/min. CHF is caused by dryout at the wall near the exit of the microchannels, which in turn is attributed to the flow reversal upstream of the microchannels. The bubbles pushed back into the inlet plenum agglomerate; the resulting flow blockage is a likely cause for the occurrence of CHF which is marked by an abrupt increase in wall temperature near the exit and an abrupt decrease in pressure drop across the microchannels. A database of 49 data points obtained from five experiments in four independent studies with water, R-113, and FC-77 as coolants was compiled and analyzed. It is found that the CHF has a strong dependence on the coolant, the flow rate, and the area upon which the flux definition is based. However, at a given flow rate, the critical heat input (total heat transfer rate to the coolant when CHF occurs) depends only on the coolant and has minimal dependence on the details of the microchannel heat sink (channel size, number of channels, substrate material, and base area). The critical heat input for flow boiling in multiple parallel microchannels follows a well-defined trend with the product of mass flow rate and latent heat of vaporization. A power-law correlation is proposed which offers a simple, yet accurate method for predicting the CHF. The thermodynamic exit quality at CHF is also analyzed and discussed to provide insights into the CHF phenomenon in a heat sink containing multiple parallel microchannels.

Experimental flow boiling heat transfer results are presented for horizontal 1.0 and 2.2 mm I.D. (internal diameter) stainless steel tubes for tests with R1234ze, a new refrigerant developed as a substitute for R134a with a much lower GWP (Global Warming Potential). These two tube diameters were chosen due the necessity to a better investigation the macro to microchannel transition boundary. The experimental campaign includes mass velocities ranging from 50 to 1500 kg m −2 s −1 , heat fluxes from 10 to 300 kW m −2 , exit saturation temperatures of 25, 31 and 35 °C, vapor qualities from 0.05 to 0.99 and heated lengths of 180 mm and 361 mm. Flow pattern characterization was performed using high-speed videos. Data for heat transfer coefficients, critical heat fluxes and flow pattern transitions were obtained. R1234ze demonstrated similar thermal performance to R134a data when running at similar conditions. For critical heat flux the correlation of Katto and Ohno (1984) best predicted the database with a mean absolute error of 6.3%. For the heat transfer coefficients, the Thome et al. (2004) three-zone model predicted the data for slug flow with 15.9% and Saitoh et al. (2007) predicted data for other flow regimes with mean absolute error of 19.4%.

The bottom configuration of a vertical finite-length cylinder is an important factor to examine the convective heat transfer by film boiling around a vertical finite-length cylinder, as the vapor generated under the bottom surface grows thicker during flowing upward along the vertical lateral surface and finally leaves the top surface as bubbles. In this study, four types of silver cylinder with a vertical lateral length equal to the diameter of 32mm were prepared for the possible combinations of bottom and top configurations: with a flat bottom and a flat top, with a flat bottom and a curved top, with a curved bottom and a flat top, and with a curved bottom and a curved top, where “flat” refers to “horizontal” and “curved” to “convex hemispherical”. Quenching experiments have been carried out for the test cylinders for saturated and subcooled water at atmospheric pressure. The initial temperature in the measurement is 600 °C. Boiling curves were obtained from the cooling curves measured using a K-type thermocouple inserted near the center on the axis of the test cylinder and the film boiling process was observed by still and high speed video cameras. The following results were obtained from the experiments using four types of test cylinder. 1. For saturated water, the test cylinders are entirely covered with a thick continuous vapor film, however, the effect of bottom configuration on film boiling heat transfer is appeared within 18% in terms of the wall heat flux averaged over the entire surface depending on the vapor fluid flow on the bottom and vertical lateral surfaces. 2. For the cylinders with a flat bottom surface, the wall heat flux averaged over the entire surface increases significantly with an increase in liquid sub cooling. This is attributed to that the convective heat transfer and the surface area ratio on the vertical lateral surface are predominant and govern the total heat transfer. 3. The effects of the cylinder top configurations on the film boiling heat transfer are small as the heat transfer on the top surface is small compared with that on the vertical lateral surface. 4. The differences between film boiling characteristics due to the bottom and top configurations are explained by examining the average heat transfer coefficient composed of the heat transfer coefficient and the surface area ratio on each surface. 5. The minimum wall superheat corresponding to the vapor-film-collapse is almost constant at 133K for four types of test cylinder in saturated water. In subcooled water, the minimum wall superheat for the cylinders with a flat bottom surface is larger than that for the cases with a convex hemispherical bottom surface.