Abstract. This paper presents a comparison of the performance of three smooth heated surfaces with different thicknesses. Analysis was carried out on an experimental setup for flowboilingheattransfer. The most important element of the setup was the test section with a rectangular minichannel, 1.7 mm deep, 16 mm wide and 180 mm long, oriented vertically. The heated element for the FC-72 Fluorinert flowing in the minichannel was designated as a Haynes-230 alloy plate (0.10 mm and 0.45 mm thick) or a Hastelloy X alloy plate (0.65 mm thick). Infrared thermography was used to measure the temperature of the outer plate surface. The local values of the heattransfer coefficient for stationary state conditions were calculated using a simple one-dimensional method. The experimental results were presented as the relationship between the heattransfer coefficients in the subcooled boiling region and the distance along the minichannel length and boiling curves. The highest local heattransfer coefficients were recorded for the surface of 0.10 mm thick heated plate at the outlet and 0.45 mm thick plate at the minichannel inlet. All boiling curves were typical in shape.
An experimental facility is developed to measure CHF and heattransfer of saturated flowboiling of R-123 in microchannels. Six parallel microchannels with cross sectional area of 1054 µm × 157 µm for each microchannel are fabricated on a copper block, and a fused silica cover is then placed on top of the copper block to serve as a transparent cover through which flow patterns and boiling phenomena could be observed. A resistive cartridge heater is used to provide a uniform heat flux to the microchannel walls. The experimental test facility is designed to accommodate test sections with different microchannel geometries. The mass flow rate, inlet temperature of R-123, and the electric current supplied to the resistive cartridge heater are controlled to provide quantitative information near the CHF condition in microchannels.
Ong and Thome (2011b) experimentally investigated flowboilingheattransfer of t hree refrigerants in channels of 1.03, 2.20 and 3.04 mm diameters. R-134a, R-236fa and R-245fa were used as working fluids in their study. The channel with higher confinement number, i.e. smaller diameter, gave heattransfer coefficients with lower dependency on heat flux. The heattransfer coefficient was also found to strongly depend on flow pattern. The coalescing bubble flow regime posed heattransfer mechanism compatible with three-zone flowboiling model proposed by Thome et al. (2004) whereas the dominance of forc ed convection was observed in the annular flow regime. The heattransfer coefficient for R-134a showed the highest dependence on h eat flux but R-245fa yielded the lowest heat flux dependency while R-236fa was positioned in between the other two refrigerants. It was noted from the authors that surface roughness played important role on micro-scale flowboiling.
The aim of this paper was to present the concept of mathematical models of heattransfer in flowboiling in an annular mini gap between the metal pipe with enhanced exterior surface and the external glass pipe. The one- and two-dimensional mathematical models were proposed to describe stationary heattransfer in the gap. A set of experimental data governed both the form of energy equations in cylindrical coordinates and the boundary conditions. The models were formulated to minimize the number of experimentally determined constants. Known temperature distributions in the enhanced surface and in the fluid helped to determine, from the Robin condition, the local heattransfer coefficients at the enhanced surface fluid contact. The Trefftz method was used to find two-dimensional temperature distributions for the thermal conductive filler layer, enhanced surface and flowing fluid. The method of temperature calculation depended on whether the area of single-phase convection ended with boiling incipience in the gap or the two-phase flow region prevailed, with either fully developed bubbly flow or bubbly-slug flow. In the two phase flow, the fluid temperature was calculated by Trefftz method. Trefftz functions for the Laplace equation and for the energy equation were used in the calculations.
It is evident that the characteristics of the flowboilingheattransfer in metal foams are strongly dependent on the metal foam structures. Mo  established an analytical model for open-cell metal foams, containing two kinds of structures, called V-type and H-type, in order to describe the microstructures of metal foams for simplification. The cylinder diameter and specific surface area of metal foams obtained by them were consistent with previous studies. Furthermore, good agreement for heattransfer coefficient was achieved between analytical results and experimental data. However, in these two models the first-class boundary conditions were presumed in which wall temperature was set constant. Krishnan [5,6] carried out a single unit cell structure to simulate heattransfer in open-cell metal foams. The void was assumed to be spherical whilst pores were located at the vertices. The total heattransfer coefficient by this model was highly accordant with experimental data. Boomsma et al.  defined an innovative microstructure of metal foams and then developed a special method for simulating flow and heattransfer in open-cell metal foams. The pressure loss and velocity distribution in foam-filled channels were investigated, which were 25% lower than those of experiment. However, these s models were established only for single phase flow conditions, and flowboiling characteristics with metal foams were not involved yet.
Since the realization in 2001 that flowboiling in microchannels can lead to high heat flux dissipation in electronics cooling applications, significant research has been directed toward improving the CHF and HTC of this configuration . It had become apparent by 2005 [33,37,38] that microchannels could not deliver on this promise, and so intense research ensued in order to overcome the difficulties in achieving this high heat flux dissipation requirement. With this background, the goal of this research was to develop a novel flowboiling microchannel system with water which would provide significant performance enhancement (high heat dissipation in excess of 1 kW/cm 2 ) for electronics cooling applications. Extensive experimental work with the new geometry has been conducted, covering a wide range of parameters so as to study their effect on the flowboiling performance. Flow régimes, underlying mechanisms, heattransfer performance as well as pressure drop modeling associated with the new geometry was undertaken. Theoretical modeling led to some key findings which enabled a surpassing of the 1 kW/cm 2 barrier with a high HTC and low pressure drop. Some of the key findings from this work are listed below:
Abstract. This paper reports an impact of selected thermal and flow parameters i.e., mass flux and inlet pressure on flowboilingheattransfer in a minichannel. Research was carried out on the experimental set up with the test section fitted with a single, rectangular and vertically oriented minichannel 1.7 mm deep. Infrared thermography was used to determine changes in the temperature on the outer side of the heated minichannel wall in the central part of the minichannel. The heated element for HFE-649 flowing in the minichannel was a thin alloy plate, made of Haynes-230. Local values of heattransfer coefficient for stationary state conditions were calculated using a simple one-dimensional method. Analysis of the results was based on experimental series obtained for the same heat flux, various mass fluxes and average inlet pressures. The experimental results are presented as the relationship between the heattransfer coefficient and the distance along the minichannel length and boiling curves. The highest local heattransfer coefficients were obtained for the lower average inlet pressure and for the highest mass flux at lower heat flux.
Dependence analysis for local heattransfer coefficients, shown in figure 5 (for vertical minichannel) and figure 6 (for horizontal minichannel) presents higher values of the heattransfer coefficients obtained from the horizontally positioned minichannel. Simultaneously, the same course was observed for the two selected calculation methods, while values of the heattransfer coefficient for a low heat flux supplied to the heating surface are similar. However, the results of the one- dimensional approach for the last settings at the maximum heat flux supplied to the heating surface (for the fully developed flowboiling) occurred to be considerably higher than in the case of the two- dimensional approach. Calculation methods have a big impact and the results differ when this method is applied. Generally, simplified one-dimensional approach, seems to be less sensitive to measurement errors in comparison with the two-dimensional one. Temperature measurement of the heating surface by liquid crystal thermography is burdened with a high error when the red colour of the surface is observed (the lowest temperatures of the active range of liquid crystal mixture). Besides, superheating T F -T sat of the heating surface appearing in eq. (1) in the
The study presented here was conducted on a modernized experimental stand. The modernization of the previous stand described in [1-4,6,7] aimed to simplify and miniaturize the setup, update the data and image acquisition system, and expand the system to enable accurate deaeration. The new setup was adapted to environmental protection requirements and prepared for carrying out a wide spectrum of experiments. The main raw data available in minichannel boilingheattransfer experiment with the application of thermosensitive liquid crystal technique included the hue distribution along the heating foil and the volumetric heat flux generated inside the foil. The new setup also enabled simultaneous observation of two-phase flow structures being generated in flowboiling in minichannels.
Abstract. The paper presents mathematical modelling of flowboilingheattransfer in a rectangular minichannel asymmetrically heated by a thin and one-sided enhanced foil. Both surfaces are available for observations due to the openings covered with glass sheets. Thus, changes in the colour of the plain foil surface can be registered and then processed. Plain side of the heating foil is covered with a base coat and liquid crystal paint. Observation of the opposite, enhanced surface of the minichannel allows for identification of the gas- liquid two-phase flow patterns and vapour quality. A two-dimensional mathematical model of heattransfer in three subsequent layers (sheet glass, heating foil, liquid) was proposed. Heattransfer in all these layers was described with the respective equations: Laplace equation, Poisson equation and energy equation, subject to boundary conditions corresponding to the observed physical process. The solutions (temperature distributions) in all three layers were obtained by Trefftz method. Additionally, the temperature of the boiling liquid was obtained by homotopy perturbation method (HPM) combined with Trefftz method. The heattransfer coefficient, derived from Robin boundary condition, was estimated in both approaches. In comparison, the results by both methods show very good agreement especially when restricted to the thermal sublayer.
Ong and Thome  have proposed a new macro-to-micro channel criterion using the confinement number Co defined by Eq. (2) according to their experimental data and observed liquid films for different flow regimes. In annular flows, for the confinement numbers greater than 1, they observed symmetric liquid film distribution along the channel wall and micro-scale flowboiling behavior. For the confinement numbers lower than about 0.3, they observed non- symmetric liquid film distribution along the channel wall. They observed isolated bubbles and bubbles coalescence and the flowboiling is microscale behavior for the confinement number greater than 1. For the confinement number is less than 0.3, they observed the plug-slug flow pattern and flowboiling exhibits macroscale channel behaviors. For confinement numbers between 1.0 and values within the range of 0.3 to 0.4, they observed the mesoscale channel behaviour according to their test conditions and test channels. However, it should be realized that the transition from macroscale-to-micoscale is a continuous and progress process which corresponds to the flow regime, heattransfer behaviors and mechanisms. Therefore, the most important point is to relate flow regimes to flowboilingheattransfer behaviours such as heattransfer, CHF and fluid behaviour such as pressure drops as pointed out by Cheng et al. . Relating flow regime behaviors to the corresponding flowboilingheattransfer and gas liquid two phase flow behaviors is a practical and effective method to develop mechanistic or phenomenal predictions for both macroscale and microscale channels due to the continuously progressive change from macroscale channels to microcale channels. This has been validated by the flow pattern based CO 2 flowboiling model of Cheng et al. [24-26], which covers both
This article presents new experimental results for two-phase flowboiling of R-134a, R-1234ze(E) and R-245fa in a micro-evaporator. The test section was made of copper and composed of 52 microchannels 163μm wide and 1560μm high with the channels separated by 178μm wide fins. The channels were 13.2mm long. There were 35 local heaters and temperature measurements arranged in a 5 × 7 array as a pseudo-CPU. The total pressure drops of the test section were below 20kPa in all cases. The wall heattransfer coefficients were generally above 10’000W/m 2 K and a function of the heat flux, vapor quality and mass flux. A newflow pattern-based prediction method for flowboilingheattransfer coefficients in microchannels was developed based on the experimental results. The new prediction method also predicted published data for four other test sections accurately, capturing the trends versus vapor quality well.
In this work, the solution of the two-dimensional inverse heattransfer problem with the use of the Beck method coupled with the Trefftz method was proposed. The experimental data of flowboilingheattransfer in a single vertical minichannel of 1.7 mm depth, heated asymmetrically, were used in calculations. The heating element for two refrigerants (FC-72 and HFE-7100) flowing in the minichannel was the plate enhanced on the side contacting with the fluid. The results were presented as infrared thermographs, heated wall temperature and heattransfer coefficient as a function of the distance from the channel inlet.
Abstract. The paper focused on flowboilingheattransfer in an annular minigap. This gap of 1 mm width was created between the metal pipe with an enhanced surface contacting fluid and the external glass pipe positioned along the same axis. The heated element for the HFE-649 flowing in the minigap was a cartridge heater. Thermocouples were used to measure the temperature of the metal pipe in the contact surface with a fluid. The local values of the heattransfer coefficient for stationary state conditions were calculated using an one-dimensional method in which the multilayer cylindrical wall was assumed to be planar. The results were presented as a function of the heattransfer coefficient along the minigap length and as boiling curves, prepared for selected values of mass flow rate and five types of the enhanced heated surface and a smooth one. Observations indicated that the highest local values of heattransfer coefficient were obtained with using the enhanced surface produced by electromachining process (spark erosion) at the saturated boiling region. The boiling curves generated for two distances from the minigap inlet have similar plots without a drop in the temperature of the heated surface characteristic for nucleation hysteresis.
In this chapter estimation of uncertainty of difference in temperature measurement received from the tested thermocouples were calculated and discussed. The difference in fluid temperature at the inlet and outlet to/out of the test section with minichannels are necessary for local heattransfer calculations according to mathematical methods based on experimental results from research on flowboilingheattransfer [9 - 12, 24 - 34].
With the rapid development of modern science and technology, flowboiling and two-phase flow in micro- and mini-channels play a significant role in cooling high heat flux in various engineering applications. Over the past decades, VLSI (vast large scale integrated) chips’ heat fluxes have already reached up to 300 W/cm 2 . The traditional cooling technology cannot meet the growing demand of heat dissipation in microelectronics and high-performance computer chips etc. Tuckerman and Pease  investigated heattransfer performance in microchannel heat sinks which have
diameter" and the effect of tube diameter on flowboilingheattransfer coefficient was characterized by the Weber number in gas phase. Results showed that this correlation could be applied to a wide range of tube diameters (0.5–11-mm-ID). In addition, the dryout point and the heattransfer characteristics after the dryout point were also investigated based on the annular flow model. A modified Chen-type correlation for the flowboilingheattransfer was developed that included the effect of tube diameter. In this correlation, the effect of tube diameter on flowboilingheattransfer was characterized by the Weber number. The correlation agreed reasonably well with experimental data for a wide range of tube diameter from 0.51 to 10.92 mm ID. From this study of paper we conclude that the modified chen type correlation better suitable for least MAE compare to kandlikar correlation and gungor and winterton correlation.
The analysis of experimental studies resulting from the author’s experience in a flowboilingheattransfer in minichannels [1-3] indicates that lower heat flux was supplied using enhanced heated surfaces compared to the smooth surface. Values of the local heattransfer coefficient have proved higher when results from the enhanced heating surfaces were noticed [2-4]. Enhanced heating surfaces were produced using various techniques, e.g. laser texturing and modified spark erosion. Some research have been undertaken to produce porous coatings to be deposited on heating surfaces using an innovative technology which are mainly used in pool boiling research [1,5,11]. It involves sintering metallic powder in the dissociated ammonia atmosphere to generate diffusion-origin pores [6,7].