Flow Boiling Heat Transfer of Refrigerant R-134a in Copper Microchannel Heat Sink

In this paper we present experimental data on heat transfer and pressure drop characteristics at flow boiling of refrigerant R-134a in a horizontal microchannel heat sink. The primary objective of this study was to experimentally establish how the local heat transfer coefficient and pressure drop correlate with the heat flux, mass flux, and vapor quality. The copper microchannel heat sink contains 21 microchannels with 335 × 930 μm² cross section. The microchannel plate and heating block were divided by the partition wall for the local heat flux measurements. Distribution of local heat transfer coefficients along the length and width of the microchannel plate was measured in the range of external heat fluxes from 50 to 500 kW/m²; the mass flux varied within 200–600 kg/m²-s, and pressure varied within 6–16 bar. The obvious impact of heat flux on the magnitude of heat transfer coefficient was observed. It showed that nucleate boiling is the dominant mechanism for heat transfer. A new model of flow boiling heat transfer, considering nucleate boiling suppression and liquid film evaporation, was proposed and verified experimentally in this paper.     

Two-Phase Flow and Boiling Heat Transfer in Small-Diameter Tubes

For the development of a high-performance heat exchanger using small channels or minichannels for air-conditioning systems, it is necessary to clarify the characteristics of vapor‐liquid two-phase flow and heat transfer of refrigerants in small-diameter tubes. In this keynote paper, the related research works that have already been performed by the author and coworkers are introduced. Based on the observations and experiments of R410A flowing in small-diameter circular and noncircular tubes with hydraulic diameter of about 1 mm, the characteristics of vapor‐liquid two-phase flow pattern and boiling heat transfer were clarified. In low quality or mass flux and low heat flux condition, in which the flow was mainly slug, the “liquid film conduction evaporation” heat transfer peculiar to small-diameter tubes prevailed and exhibited considerably good heat transfer compared to nucleate boiling and forced convection evaporation heat transfer. The effects of the tube cross-sectional shape and flow direction on the heat transfer primarily appeared in the region of the “liquid film conduction evaporation” heat transfer. A new heat transfer correlation considering all of three contributions has been developed for small circular tubes.     

Flow Boiling in an Inclined Channel With Downward-Facing Heated Upper Wall

Wall boiling and bubble population balance equations combined with a two-fluid model are employed to predict boiling two-phase flow in an inclined channel with a downward-facing heated upper wall. In order to observe the boiling behavior on the inclined, downward-facing heated wall, a visualization experiment was carried out with a 100 mm × 100 mm of the cross section, 1.2-m-long rectangular channel, inclined by 10° from the horizontal plane. The size of the heated wall was 50 mm by 750 mm and the heat flux was provided by Joule heating using DC electrical current. The temperatures of the heater surface were measured and used in calculating heat transfer coefficients. The wall superheat for 100 kW/m² heat flux and 200 kg/m²s mass flux ranged between 9.3°C and 15.1°C. High-speed video images showed that bubbles were sliding, continuing to grow, and combining with small bubbles growing at their nucleation sites in the downstream. Then large bubbles coalesced together when the bubbles grew too large to have a space between them. Finally, an elongated slug bubble formed and it continued to slide along the heated wall. For these circumstances of wall boiling and two-phase flow in the inclined channel, the existing wall boiling model encompassing bubble growth and sliding was improved by considering the influence of large bubbles near the heated wall and liquid film evaporation under the large slug bubbles. With this improved model, the predicted wall superheat agreed well with the experimental data, while the RPI model largely overpredicted the wall superheat.     

Convection and evaporation of microlayer underneath bubbles under microgravity

The multidimensional heat transfer and fluid flow in the microlayer region below a vapor bubble formed during boiling in microgravity are investigated by numerically solving the Navier–Stokes equations with the energy equation. The flow is driven by Marangoni flow due to the surface tension gradient along the bubble surface that results from the temperature gradient. The model also includes condensation and evaporation at the bubble surface. The flow field and heat transfer are calculated for microlayer thicknesses from 0.01 mm to 10 mm to investigate the effect of microlayer thickness. The results show that the velocities are small and have only a small effect on the temperature distribution as compared to the solution for pure conduction in the liquid. Natural convection is shown to have a negligible effect on heat transfer. For less than ideal evaporative heat transfer at the bubble interface, Marangoni convection caused the heat transfer to increase several percent. The flow in the microlayer is shown to agree with the lubrication analogy only for thin, relatively flat interfaces. © 2000 Scripta Technica, Heat Trans Asian Res, 30(1): 1–10, 2001     

Unified theoretical prediction of fully developed nucleate boiling and critical heat flux based on a dynamic microlayer model

A new dynamic microlayer model has been proposed to predict theoretically the heat flux in fully developed nucleate boiling regions including critical heat flux (CHF). In this model, the heat transfer with boiling is mainly attributed to the evaporation of the microlayers which are periodically formed while the individual bubbles are forming. Since the initial microlayer thickness becomes thinner with the increase of wall superheat, both the local evaporation and the partial dryout speed of the microlayer increase. As a result, the time-averaged heat flux during the period of individual bubble has a maximum point, the CHF, at the predicted continuous boiling curve.     

High-Speed Visualization of Bubble Behaviors for Pool Boiling of R-141b

A visualization study on the behavior of bubbles has been carried out for pool boiling of R141b on a horizontal transparent heater at pressure 0.1 MPa. The behaviors of bubbles were recorded by a high-speed camera placed beneath the heater surface. The departure diameter, departure time of bubbles and nucleation site density at different heat flux were obtained. The visualization results show that bubble departure diameter and departure time decrease , while the nucleation site density increases as the heat flux increases. It is also observed that there is no liquid recruited into the microlayer in the experiment. Based on the experimental results, boiling curve for R141b was predicted by using the dynamic microlayer model. As a result, the agreement between the predictive result based on the dynamic microlayer model and the experiment data for boiling curve of R141b is good at high heat flux.     

Two-Phase Heat Sinks with Microporous Coating

To minimize flow boiling instabilities in two-phase heat sinks, two different types of microporous coatings were developed and applied on mini- and small-channel heat sinks and tested using degassed R245fa refrigerant. The first coating was epoxy based and was sprayed on heat sink channels, while the second coating was formed by sintering copper particles on heat sink channels. Minichannel heat sinks had overall dimensions 25.4 mm × 25.4 mm × 6.4 mm and 12 rectangular channels with a hydraulic diameter 1.7 mm and a channel aspect ratio of 2.7. Small-channel heat sinks had the same overall dimensions, but only three rectangular channels with hydraulic diameter 4.1 mm and channel aspect ratio 0.6. The microporous coatings were found to minimize parallel channel instabilities for minichannel heat sinks and to reduce the amplitude of heat sink base temperature oscillations from ～6°C to slightly more than 1°C. No increase in pressure drop or pumping power due to the microporous coating was measured. The minichannel heat sinks with porous coating had on average 1.5 times higher heat transfer coefficient than uncoated heat sinks. Also, the small-channel heat sinks with the “best” porous coating had on average 2.5 times higher heat transfer coefficient and the critical heat flux was 1.5 to 2 times higher compared with the uncoated heat sinks.     

Enhancement of nucleate boiling on composite surfaces

A composite heating surface composed of materials with different thermal conductivities can be expected to enhance heat transfer in nucleate boiling. This is because the end surface, with higher conductivity, will attain a higher temperature and as a result will serve to provide preferential nucleation sites. To confirm this idea, several composite surfaces were fabricated by uniaxially imbedding thin copper cylinders in the heat flow direction on a stainless steel circular plate 30 mm in diameter and 5 mm thick. The imbedded copper cylinders ranged from 1 mm to 4 mm in diameter and one to 77 in number. The heat transfer performance of these composite surfaces was investigated for pool boiling of saturated water at atmospheric pressure. It was confirmed that the copper cylinder surfaces exposed to water functioned as local hot spots to initiate preferential nucleate boiling, leading to higher boiling heat transfer coefficients than those on a homogeneous stainless steel surface. The measured void fraction above the heating surface verified intensive bubble generation on the surface of the copper cylinders. This situation continued up to a certain heat flux level and was then followed by nucleation on the mother surface of stainless steel around the copper cylinders. A numerical analysis of heat conduction within a composite wall simulated the temperature distribution within the wall and the variation in surface heat flux at the time of boiling incipience. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(3): 216–228, 1998     

Interactions Between Parallel Unevenly Heated Minichannels During Flow Boiling of R134A

Transient pressure drop of individual channels during flow boiling of R134a in four 0.54 mm square parallel minichannels was experimentally studied in this work. The design of the test section enabled the experimenter to control and to vary heat flux independently in each channel in the range from 3.82 to 18.66 kW/m² at five different overall flow rates from 86 to 430 kg/m²-s. Flow rate fluctuation in parallel channels due to the formation of bubbles under the nonuniform heat flux conditions caused significant oscillations in local pressure drop. Statistical analysis indicated that the pressure drop signal was normally distributed when boiling was stable with no incoming flow disturbance. Pressure drop distribution was highly skewed and multimodal when significant evaporation rate at low mass fluxes led to rapid annular flow formation, reducing the free flow of incoming fluid. Cross-correlation analysis revealed a strong interaction between minichannels having the highest heat flux difference among the set of channels. The least heated channel was more sensitive to the fluctuations in other channels. Cross-correlation between the most heated channel and the adiabatic one was estimated to be 39% when the total flow rate was the lowest, 86 kg/m²-s. The power of the relationship between channels dropped significantly as the flow rate increased. Less than 5% of data points could be considered cross-correlated at the highest flow rate of 430 kg/m²-s. Increasing the two-phase pressure drop across each channel caused higher resistance to the incoming disturbances and led to less interchannel interaction. This study of the channels interaction in a system of parallel, nonuniformly heated minichannels can be used as a tool to identify and quantify instabilities and reversed flow conditions.     

Heat Transfer Correlations for Single-Phase Flow,Condensation, and Boiling in Microfin Tubes

Experimental single-phase, condensation and flow boiling heat transfer data from the literature and our previous studies were collected to evaluate existing heat transfer correlations for microfin tubes of different geometries. The Ravigururajan and Bergles correlation modified by using the hydraulic diameter proposed by Li et al. (2012) can predict single-phase heat transfer data relatively well. Among the four reviewed condensation heat transfer correlations, the Yu and Koyama (1998) correlation presents the best prediction. However, all the four condensation correlations are prone to overpredict the carbon dioxide data. For flow boiling in microfin tubes, the general semiempirical correlation developed by Wu et al. (2013), applicable for intermittent and annular flow patterns, is the most reliable predictive method among the five evaluated correlations. It can predict 90% of the overall 754 data points within a ±30% error band, with a mean absolute deviation and a standard deviation equal to 18.2% and 21.9%, respectively, covering pure halogenated refrigerants, near azeotropic refrigerant mixtures, and carbon dioxide with the following applicable range: fin root diameter 2.1 to 14.8 mm, mass flux 100 to 800 kg/m²s, heat flux 4.5 to 59 kW/m², and reduced pressure 0.07 to 0.7.     

Theoretical studies on transient pool boiling based on microlayer model

An analytical model for transient pool boiling heat transfer was developed in this study. The boiling curves of the transient boiling were obtained based on the microlayer model proposed by the authors and the mechanism of transition from the non-boiling regime to film boiling, i.e., direct transition was theoretically examined. Since the nucleate boiling heat flux is mainly due to the evaporation of the microlayer and its initial thickness decreases rapidly with increasing superheat, the duration of nucleate boiling is markedly decreased as the incipient boiling superheat is increased. It is found that the direct transition is closely connected to the rapid dryout of the microlayer which occupies almost the whole surface at high wall superheat.     

Flow boiling of liquid nitrogen in micro-tubes: Part II – Heat transfer characteristics and critical heat flux

This paper is the second portion of a two-part study concerning the flow boiling of liquid nitrogen in the micro-tubes with the diameters of 0.531, 0.834, 1.042 and 1.931 mm. The contents include the heat transfer characteristics and critical heat flux (CHF). The local wall temperatures are measured, from which the local heat transfer coefficients are determined. The influences of heat flux, mass flux, pressure and tube diameter on the flow boiling heat transfer coefficients are investigated systematically. Two regions with different heat transfer mechanism can be classified: the nucleate boiling dominated region for low mass quality and the convection evaporation dominated region for high mass quality. For none of the existed correlations can predict the experimental data, a new correlation expressed by Co, Bo, We, K_p and X is proposed. The new correlation yields good fitting for 455 experimental data of 0.531, 0.834 and 1.042 mm micro-tubes with a mean absolute error (MAE) of 13.7%. For 1.931 mm tube, the flow boiling heat transfer characteristics are similar to those of macro-channels, and the heat transfer coefficient can be estimated by Chen correlation. Critical heat flux (CHF) is also measured for the four tubes. Both the CHF and the critical mass quality (CMQ) are higher than those for conventional channels. According to the relationship that CMQ decreases with the mass flux, the mechanism of CHF in micro-tubes is postulated to be the dryout or tear of the thin liquid film near the inner wall. It is found that CHF increases gradually with the decrease of tube diameter.     

Boiling Heat Transfer for Freon R21 in Rectangular Minichannel

In this paper, we study the boiling heat transfer of upward flow of R21 in a vertical mini-channel with a size of 1.6 × 6.3 mm. The heat transfer coefficient was measured as a function of heat flux for a wide range of vapor quality and for two levels of mass flow rate, G = 215 kg/m²s and G = 50 kg/m²s. The standard deviation of wall superheat over channel perimeter and in time was determined from the measurement of the wall temperature along the channel perimeter. Different heat transfer mechanisms were revealed depending on flow patterns. The main heat transfer mode for large mass flux is convective boiling. We also figure out the mode when the evaporation of thin liquid films makes the essential contribution to heat transfer. The modified models of Liu & Winterton and Balasubramanian & Kandlikar describe the experimental data well for regime when the convective boiling makes the main contribution to the heat transfer.     

Maximum heat flux in relation to quenching of a high temperature surface with liquid jet impingement

Experimental investigation has been conducted for quenching of hot cylindrical blocks made of copper, brass and steel with initial block temperature 250–400 °C by a subcooled water jet of diameter of 2 mm. The subcooling was from 5 to 80 K and the jet velocity was from 3 to 15 m/s. After impingement, the jet stagnates for a certain period of time in a small region near the centre and then the wetting front starts moving outwards. During this movement, when the surface temperature at the wetting front drops to 120–200 °C, the surface heat flux reaches its maximum value due to forced convection nucleation boiling. The maximum heat flux is a strong function of the position on the hot surface, jet velocity, block material properties and jet subcooling. A new correlation for maximum heat flux is proposed.     

Two-phase cooling characteristics of a saturated free falling circular jet of HFE7100 on a heated disk: Effect of jet length

Direct jet impingement boiling heat transfer operating at low flow rates is of great interest for the localized moderate heat fluxes from the targets with delicate mechanical structure, where the aggressive techniques such as high-speed jets are not suitable. Boiling heat transfer from an upward facing disk targeted by a falling jet was studied experimentally at different volumetric flow rates and various jet lengths. The working fluid was chosen to be the dielectric liquid HFE7100 and the heated spot was an 8-mm diameter disk. Using previous CHF correlations in their original form, valid at very low volumetric flow rates, results in large disagreements since it was found that variation in the jet length changes the boiling characteristics. It is demonstrated that although the circular hydraulic jump formation within the heater diameter may suppress the heat transfer under certain conditions, moving the jet closer to the target may significantly improve the boiling curves at the critical heat flux (CHF) regime. At low flow rates, the CHF increases as the jet length decreases while for moderate and high flow rates the boiling curves show approximately a universal behavior for different jet lengths. For such low flow rates, the effect of jet length on boiling curves was shown to be related to the variation of the cross section of the falling jet and the formation of hydraulic jump at radial distances smaller than the heater diameter. The current CHF results for different jet lengths are correlated by including the effect of jet length in the previous correlation proposed by Sharan and Lienhard.     

R1234yf Flow Boiling Heat Transfer Inside a 2.4-mm Microfin Tube

ABSTRACT

This paper presents an experimental study on R1234yf flow boiling inside a mini microfin tube with an inner diameter at the fin tip of 2.4 mm. R1234yf is a new refrigerant with an extremely low global warming potential (GWP <1), proposed as a possible substitute for the common R134a, whose GWP is about 1300. The mass flux was varied between 375 and 940 kg m^?2 s^?1, heat flux from 10 to 50 kW m^?2, and vapor quality from 0.1 to 1. The saturation temperature at the inlet of the test section was kept constant and equal to 30°C. The wide range of operative test conditions permitted highlighting the effects of mass flux, heat flux, and vapor quality on the thermal and hydraulic behavior during the flow boiling mechanism inside such a mini microfin tube. The results show that at low heat flux the phase-change process is mainly controlled by two-phase forced convection, and at high heat flux by nucleate boiling. The two-phase frictional pressure drop increases with increasing both mass velocity and vapor quality. Dry-out was observed only at the highest heat flux, at vapor qualities of around 0.94–0.95.     

Evaporation Heat Transfer Coefficients of R-446A

ABSTRACT

The present study investigated the evaporation heat transfer coefficients of R-446A, as a low global warming potential alternative refrigerant to R-410A. The evaporation heat transfer coefficients were obtained by measuring the wall temperature of a straight stainless tube and refrigerant pressure. The heat transfer coefficients were measured for the quality range from 0.05 to 0.95, the mass flux from 100 to 400 kg/m²s, heat flux from 10 to 30 kW/m², and saturation temperature from 5 to 10°C. The evaporation heat transfer coefficient of R-410A was verified by comparing the measured evaporation heat transfer coefficient with the value predicted by the existing correlation. The evaporation heat transfer coefficient of R-446A was measured using a proven experimental apparatus. When the heat flux was 10 kW/m², the evaporation heat transfer coefficient of R-446A was always higher than that of R-410A. But, when the heat flux was 30 kW/m², the evaporation heat transfer coefficient of R-446A was measured to be lower than that of R-410A near the dry-out point. The effect of the tube diameter on the R-446A evaporation heat transfer coefficient was negligible. The effect of saturation pressure on the evaporation heat transfer coefficient was prominent in the low quality region where the nucleate boiling was dominant.     

Confined Submerged Jet Impingement Boiling of Subcooled FC-72 over Micro-Pin-Finned Surfaces

Heat transfer characteristics of confined submerged jet impingement boiling of air-dissolved FC-72 on heated micro-pin-finned surfaces are presented. The dimension of the silicon chips is 10 × 10 × 0.5 mm³ (length × width × thickness) on micro-pin-fins with the four dimensions of 30 × 30 × 60 μm³, 50 × 50 × 60 μm³, 30 × 30 × 120 μm³, and 50 × 50 × 120 μm³ fabricated by using the dry etching technique. For comparison, experiments of jet impinging on a smooth surface were also conducted. The results have shown that submerged jet impingement boiling gives a large heat transfer enhancement compared with pool boiling, and all micro-pin-fins showed better heat transfer performance than a smooth surface. The effects of jet Reynolds number, jet inlet subcooling, micro-pin-fins, and nozzle-to-surface distance on jet impingement boiling heat transfer were explored. For micro-pin-fins, the maximum allowable heat flux increases with jet Reynolds number and subcooling. The largest value of the maximum allowable heat flux of micro-pin-fins by submerged jet impingement boiling is 157 W/cm², which is about 8.3 times as large as that for the smooth surface in pool boiling. Also, Nusselt number has a strong dependence on Reynolds number.     

Enhanced Pool Boiling Performance of Microchannel Patterned Surface with Extremely Low Wall Superheat through Capillary Feeding of Liquid

ABSTRACT

The pool boiling performance plays a key role in the development of high heat flux dissipating applications. The high critical heat flux and low wall superheat are two of the critical factors that affect the long-term life of devices. In this paper, enhanced pool boiling performance can be achieved by well-designed microchannels in copper surfaces using a precision diamond dicing method. The microchannel patterned surface with the channel length of 0.4 mm obtains a critical heat flux of 169.8 W/cm², which has a 193% enhancement compared to the plain surface. Besides, the extremely low wall superheat of 3 K has been achieved, and thus the heat transfer coefficient reaches 51.8 W/cm²·K, about 738% larger than that of the plain surface. Herein, the microcavity has increased the nucleation site, the surface can promote the bubbles escape, and then the channel can continuously supply the liquid. Hence, the extremely low wall superheat at high heat flux occurs due to the rapid bubble departure and enhanced capillary feeding of liquid replenishment to active nucleation sites on the surface. The above results provide an effective way for the realization of high-performance two-phase microchannel patterned heat sinks via optimizing the microstructure geometry.     

Microlayer thickness under a vapor bubble on a cylindrical probe

The thickness of a liquid microlayer underneath a vapor bubble on a heated, cylindrical probe was determined by simultaneously solving the fourth‐order differential equation for the microlayer thickness that incorporates the momentum and energy equations in the microlayer in conjunction with the pressure distribution in the microlayer and the evaporative heat flux at the interface. The analysis also considers the temperature gradient along the probe due to heat transfer in the probe. The results show that the microlayer on a cylindrical surface is very thin and short except for very low probe surface temperatures, superheated less than 1 K. The microlayer size and the evaporative heat flux both decrease rapidly as the surface temperature increases. The results show that most of the evaporation occurs along the curved portion of the interface. © 2000 Scripta Technica, Heat Trans Asian Res, 29(3): 193–203, 2000