The effect of liquid film evaporation on flow boiling heat transfer in a micro tube

Flow boiling in micro channels is attracting large attention since it leads to large heat transfer area per unit volume. Generated vapor bubbles in micro channels are elongated due to the restriction of channel wall, and thus slug flow becomes one of the main flow regimes. In slug flow, sequential bubbles are confined by the liquid slugs, and thin liquid film is formed between tube wall and bubble. Liquid film evaporation is one of the main heat transfer mechanisms in micro channels and liquid film thickness is a very important parameter which determines heat transfer coefficient. In the present study, liquid film thickness is measured by laser focus displacement meter under flow boiling condition and compared with the correlation proposed for an adiabatic flow. The relationship between liquid film thickness and heat transfer coefficient is also investigated. Initial liquid film thickness under flow boiling condition can be predicted well by the correlation proposed under adiabatic condition. Under flow boiling condition, liquid film surface fluctuates due to high vapor velocity and shows periodic pattern against time. Frequency of periodic pattern increases with heat flux. At low quality, heat transfer coefficients calculated from measured liquid film thickness show good accordance with heat transfer coefficients obtained directly from wall temperature measurements.     

Contribution of thin-film evaporation during flow boiling inside microchannels

Flow boiling through microchannels is characterized by nucleation and growth of vapor bubbles that fill the entire channel cross-sectional area. As the bubbles nucleate and grow inside the microchannel, a thin film of liquid or a microlayer gets trapped between the bubbles and the channel walls. The heat transfer mechanism present at the channel walls during flow boiling is studied numerically. It is then compared to the heat transfer mechanisms present during nucleate pool boiling and in a moving evaporating meniscus. Increasing contact angle improved wall heat transfer in case of nucleate boiling and moving evaporating meniscus but not in the case of flow boiling inside a microchannel. It is shown that the thermal and the flow fields present inside the microchannel around a bubble are fundamentally different as compared to nucleate pool boiling or in a moving evaporating meniscus. It is explained why thin-film evaporation is the dominant heat transfer mechanism and is responsible for creating an apparent nucleate boiling effect inside a microchannel.     

Single-phase and two-phase heat transfer characteristics of low temperature hybrid micro-channel/micro-jet impingement cooling module

This study examines the single-phase and two-phase cooling performance of a hybrid micro-channel/micro-jet impingement cooling scheme using HFE 7100 as working fluid. This scheme consists of supplying coolant from a series of jets that deposit liquid into the micro-channels. A single-phase numerical scheme that utilizes the k–ε turbulent model and a method for determining the extent of the laminarized wall layer shows very good predictions of measured wall temperatures. It is shown jet velocity has a profound influence on single-phase cooling performance. High jet velocities enable jet fluid to penetrate the axial micro-channel flow and produce a strong impingement effect at the wall. On the other hand, the influence of jets at low jet velocities is greatly compromised compared to the micro-channel flow. During nucleate boiling, vapor layer development along the micro-channel in the hybrid module is fundamentally different from that encountered in conventional micro-channels. Here, subcooled jet fluid produces repeated regions of bubble growth followed by bubble collapse, rather than the continuous growth common to conventional micro-channel flow. By reducing void fraction along the micro-channel, the hybrid scheme contributes greater wall temperature uniformity. Increasing subcooling and/or flow rate delay the onset of boiling to higher heat fluxes and higher wall temperatures, and also increase critical heat flux considerably. A nucleate boiling heat transfer coefficient correlation is developed that fits the present data with a mean absolute error of 6.10%.     

Transition and flow boiling heat transfer inside a horizontal tube

Experiments on transition and flow boiling heat transfer with refrigerant R114 inside a horizontal tube were performed at bubble flow, critical heat flux and in the transition region between bubble flow and film boiling at mass fluxes between 1200 and 4000 kg/m² s and in the pressure range between 5 and 15 bar. In comparison with pool boiling bubble flow heat transfer depends essentially on the mass flow rates and on the vapor quality. The critical heat flux depends less on the temperature difference than in pool boiling heat transfer and exhibits a maximal and a minimal value as a function of the pressure. The critical heat flux increases with mass flow rate as already shown by Collier. In the region of transition boiling the heat flux over the difference between wall and saturation temperature approaches a horizontal curve. Therefore in this region an evaporator may always be operated under stable conditions and burn out does not occur.     

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.     

A study on boiling in a horizontal channel with a rectangular section (in the case of a heated bottom wall)

The characteristics of boiling in a horizontal channel with changing conditions of the length of the heated wall and the channel height have been studied experimentally. Behavior of bubbles on the heated wall and growth of bubbles in the channel were observed by a high‐speed camera. As a result, the behavior of the growth of bubbles which was classified into three types according to channel height had an influence on the time variation of the degree of superheat, heat transfer, and burnout heat flux in the channel. When the liquid on the bottom wall became thin, nucleate boiling with a vapor dome was observed on the heated wall. © 2000 Scripta Technica, Heat Trans Asian Res, 29(6): 459–472, 2000     

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.     

Subcooled flow boiling and microbubble emission boiling phenomena in a partially heated microchannel

A simultaneous visualization and measurement study has been carried out to investigate subcooled flow boiling and microbubble emission boiling (MEB) phenomena of deionized water in a partially heated Pyrex glass microchannel, having a hydraulic diameter of 155 μm, which was integrated with a Platinum microheater. Effects of mass flux, inlet water subcooling and surface condition of the microheater on subcooled flow boiling in microchannels are investigated. It is found that MEB occurred at high inlet subcoolings and at high heat fluxes, where vapor bubbles collapsed into microbubbles after contacting with the surrounding highly subcooled liquid. In the fully-developed MEB regime where the entire microheater was covered by MEB, the mass flux, the inlet water subcooling and the heater surface condition have only small effects on the boiling curves. The occurrence of MEB in microchannel can remove a large amount of heat flux, as high as 14.41 MW/m² at a mass flux of 883.8 kg/m² s, with only a moderate rise in wall temperature. Therefore, MEB is a very promising method for cooling of microelectronic chips. Heat transfer in the fully-developed MEB in the microchannel is presented, which is compared with existing subcooled flow boiling heat transfer correlations for macrochannels.     

Subcooled and saturated boiling of ammonia–lithium nitrate solution in a plate-type generator for absorption machines

This paper presents the experimental heat transfer evaluation during subcooled and saturated boiling of ammonia–lithium nitrate solution in a fusion plate heat exchanger, acting as a vapor generator under operating conditions representative of single-effect absorption machines. The solution flow rate and outlet temperature were modified in the ranges of 0.041–0.083 kg/s and 78–97 °C, respectively. The region where vapor bubbles begin to arise is estimated using a correlation for the wall superheat required for the onset of nucleate boiling. Results show that subcooled boiling is present in the generator. The initial boiling temperature is about 3.1 °C lower than the saturation temperature. The influence of the heat and mass fluxes on the boiling heat transfer coefficient is analyzed. The paper offers a correlation for the Nusselt number, including the subcooled and saturated boiling regions.     

On the dynamics and heat transfer of bubble train in micro-channel flow boiling

The dynamics and heat transfer characteristics of flow boiling bubble train moving in a micro channel is studied numerically. The coupled level set and volume of fluid (CLSVOF) is utilized to track interface and a non-equilibrium phase change model is applied to calculate the interface temperature as well as heat flux jump. The working fluid is R134a and the wall material is aluminum. The fluid enters the channel with a constant mass flux (335 kg/m² 1 s), and the boundary wall is heated with constant heat flux (14 kW/m²). The growth of bubbles and the transition of flow regime are compared to an experimental visualization. Moreover, the bubble evaporation rate and wall heat transfer coefficient have been examined, respectively. Local heat transfer is significantly enhanced by evaporation occurring vicinity of interface of the bubbles. The local wall temperature is found to be dependent on the thickness of the liquid film between the bubble train and the wall.     

Monodisperse Spray Cooling of Small Surface Areas at High Heat Flux

Spray cooling is an effective method to remove high heat fluxes from electronic components. To understand the physical mechanisms, this work studies heat transfer rates from single and dual nozzle distilled water sprays on a small heated surface (1.3 mm × 2 mm). Thermal ink jet atomizers generate small droplets, 33 μm diameter, at known frequencies, leading to controlled spray conditions with a monodisperse stream of droplets interacting with the hot surface. Of particular interest in this work is the dissipated heat flux and its relation to the liquid film thickness, the surface superheat, and the cooling mass flow rate. Experimental results show the heat flux scales to the cooling mass flow rate. In comparison to published spreading–splashing correlations, these experiments indicate that the drops impinge on the liquid film and spread without generating splashing, leading to high-efficiency stable heat transfer. Surface temperatures range from 120 to 140°C. In addition, the liquid film thickness is investigated in relation to the heater superheat and a stable thin film is seen at superheats beyond 20°C. The efficiency of the spray system is inversely related to the film thickness and may be due to ejection of liquid from the surface due to bursting of vapor bubbles.     

Fundamental consideration of wall heat partition of vertical subcooled boiling flows

Improved wall heat flux partitioning accounting sliding bubbles and a mechanistic model that incorporates the fundamental consideration of bubble frequency during low-pressure subcooled flow boiling is presented. A model considering the forces acting on departing bubbles at the heated surface is employed. Coupled with a three-dimensional two-fluid and population balance equations based on the modified MUSIG (MUltiple-SIze-Group) model, the behavior of an upward forced convective subcooled boiling flows in a vertical annular channel is simulated. Comparison of model predictions against local and axial measurements (heat fluxes ranged from 152.9 to 705.0 kW/m²) is made for the void fraction, Sauter mean bubble diameter and interfacial area concentration covering a range of different mass and heat fluxes and inlet subcoolings. Good agreement is achieved between the predicted and measured profiles. Reasonable agreement with recent experimental measurements is also attained for the predicted growth and waiting times of bubble frequency at particular local wall superheat and subcooling temperatures.     

Wall heat flux partitioning during subcooled forced flow film boiling of water on a vertical surface

Subcooled flow film boiling experiments were conducted on a vertical flat plate, 30.5 cm in height, and 3.175 cm wide with forced convective upflow of subcooled water at atmospheric pressure. Data have been obtained for mass fluxes ranging from 0 to 700 kg/m²s, inlet subcoolings ranging from 0 to 25 °C and wall superheats ranging from 200 to 400 °C. Correlations for wall heat transfer coefficient and wall heat flux partitioning were developed as part of this work. These correlations derive their support from simultaneous measurements of the wall heat flux, fluid temperature profiles, liquid side heat flux and interfacial wave behavior during steady state flow film boiling. A new correlation for the film collapse temperature was also deduced by considering the limiting case of heat flux to the subcooled liquid being equal to the wall heat flux. The premise of this deduction is that film collapse under subcooled conditions occurs when there is no net vapor generation. These correlations have also been compared with the data and correlations available in the literature.     

Effects of enhanced surfaces and surface orientation on nucleate and film boiling heat transfer in R-11

A study of pool boiling heat transfer in R-11 is reported as a function of surface characterization and orientation. Two specially prepared metal coated surfaces (UNB#1, UNB#2) and a flat copper surface were subjected to heat fluxes up to 180 kW m⁻² with surface orientations varying from horizontally facing upward (0°), to vertical (90°), to horizontally facing downward (180°). The resulting nuleate boiling curves display considerable boiling hysteresis and enhanced surfaces show 2–3 times better heat transfer than a plain surface. Rohsenow's nucleate boiling equation is used to correlate the data and modified to account for the effects of surface characterization and orientation. In film boiling, enhanced surfaces also reveal better heat transfer characteristics and the role of surface orientation on the motion and stability of the vapor film is clarified.     

Investigation of heat transfer and bubble dynamics in a boiling thin liquid film flowing over a rotating disk

Nucleate boiling heat transfer and bubble dynamics in a thin liquid film on a horizontal rotating disk were studied. A series of experiments were conducted to determine the heat transfer coefficient on the disk. At low rotation and flow rates, vigorous boiling increased the heat transfer coefficients above those without boiling. Higher rotational speeds and higher flow rates increased the heat transfer coefficient and suppressed boiling by decreasing the superheat in the liquid film. The flow field on the disk, which included supercritical (thin film) flow upstream of a hydraulic jump, and subcritical (thick film) flow downstream of a hydraulic jump, affected the type of bubble growth. Three types of bubble growth were identified. Vigorous boiling with large, stationary bubbles were observed in the subcritical flow. Supercritical flow produced small bubbles that remained attached to the disk and acted as local obstacles to the flow. At low rotational rates, the hydraulic jump that separated the supercritical and subcritical regions produced hemispherical bubbles that protruded out of the water film surface and detached from the disk, allowing them to slide radially outward. A model of the velocity and temperature of the microlayer of water underneath these sliding bubbles indicated that the microlayer thickness was approximately 1/25th of that of the surrounding water film. This microlayer is believed to greatly enhance the heat transfer rate underneath the sliding bubbles.     

Cross Vapor Stream Effect on Falling Film Evaporation in Horizontal Tube Bundle Using R134a

The falling film evaporation of R134a with nucleate boiling outside a triangular-pitch (2-3-2-3) tube bundle is experimentally investigated, and the effects of saturation temperature, film flow rate and heat flux on heat transfer performance are studied. To study the effect of cross vapor stream on the falling film evaporation, a novel test section is designed, including the tube bundle, liquid and extra vapor distributors. The measurements without extra vapor are conducted at the saturation temperature of 6, 10 and 16°C, film Reynolds number of 220 to 2650, and heat flux of 20 to 60 kWm^?2. Cross vapor stream effect experiments are operated at three heat fluxes 20, 30, and 40 kWm^?2 and two film flow rates of 0.035 and 0.07 kgm^?1s^?1, and the vapor velocity at the smallest clearance in the tube bundle varies from 0 to 2.4 ms^?1. The results indicate that: film flow rate, heat flux and saturation temperature significantly influence the heat transfer; the cross vapor stream either promote or inhibit the falling film evaporation, depending on the tube position, film flow rate, heat flux and vapor velocity.     

Pool Boiling Heat Transfer in Diluted Water/Glycerol Binary Solutions

Pool boiling heat transfer in water/glycerol binary solutions has been experimentally investigated on a horizontal rod heater. The experiments have been performed at various concentrations (zero to 35% mass glycerol) and heat fluxes up to 92 kW m^?2 at atmospheric pressure. The experimental values of boiling heat transfer coefficient have been compared to main existing correlations. It has been shown that the various predictions are significantly inconsistent. Based on the high difference between relative volatilities of water and glycerol, a simple model has been proposed to predict the boiling heat transfer coefficient. The applicability of this model is limited to low concentrations of glycerol and medium/low heat fluxes; however, the predictions are accurate. The proposed model is anticipated to be extendable to other binary systems in which the vapor pressure of one constituent is considerably higher when compared to the other component.     

Dynamic surface effect on boiling of aqueous surfactant solutions

Boiling of aqueous surfactant solutions is known to be drastically different from that of pure water. The experimental results showed that a small amount of surfactant additive makes the nucleate boiling heat transfer coefficient, h_b, considerably higher, and that there is an optimum additive concentration for higher heat fluxes. Beyond this optimum point, further increase in additive concentration makes h_b lower. In this work, the effect of surfactant additive on boiling is attributed to the departure from equilibrium surface tension which is produced by extension of the vapor-liquid interface during growth and coalescence of vapor bubbles in the vicinity of boiling surface. The surface tension of a surfactant solution is higher than the static value because the surfactant component cannot diffuse to the adsorbed layer promptly. This dynamic surface effect can be expressed by the Y (≡c(dσ/dc)²) value which is shown to be a representation of the elasticity of an adsorbed film as well. The similarity between the increment of h_b due to the addition of surfactant and the Y curves indicates that the dynamic surface effect may play an important role in the boiling process of aqueous surfactant solutions.     

Microchannel size effects on local flow boiling heat transfer to a dielectric fluid

Heat transfer with liquid–vapor phase change in microchannels can support very high heat fluxes for use in applications such as the thermal management of high-performance electronics. However, the effects of channel cross-sectional dimensions on the two-phase heat transfer coefficient and pressure drop have not been investigated extensively. In the present work, experiments are conducted to investigate the local flow boiling heat transfer of a dielectric fluid, Fluorinert FC-77, in microchannel heat sinks. Experiments are performed for mass fluxes ranging from 250 to 1600 kg/m² s. Seven different test pieces made from silicon and consisting of parallel microchannels with nominal widths ranging from 100 to 5850 μm, all with a nominal depth of 400 μm, are considered. An array of temperature sensors on the substrate allows for resolution of local temperatures and heat transfer coefficients. The results of this study show that for microchannels of width 400 μm and greater, the heat transfer coefficients corresponding to a fixed wall heat flux as well as the boiling curves are independent of channel size. Also, heat transfer coefficients and boiling curves are independent of mass flux in the nucleate boiling region for a fixed channel size, but are affected by mass flux as convective boiling dominates. A strong dependence of pressure drop on both channel size and mass flux is observed. The experimental results are compared to predictions from a number of existing correlations for both pool boiling and flow boiling heat transfer.     

On the rise paths of single vapor bubbles after the departure from nucleation sites in subcooled upflow boiling

A photographic study was carried out for the subcooled flow boiling of water to elucidate the rise characteristics of single vapor bubbles after the departure from nucleation sites. The test section was a transparent glass tube of 20 mm in inside diameter and the flow direction was vertical upward; liquid subcooling was parametrically changed within 0–16 K keeping system pressure and liquid velocity at 120 kPa and 1 m/s, respectively. The bubble rise paths were analyzed from the video images that were obtained at the heat flux slightly higher than the minimum heat flux for the onset of nucleate boiling. In the present experiments, all the bubbles departed from their nucleation sites immediately after the inception. In low subcooling experiments, bubbles slid upward and consequently were not detached from the vertical heated wall; the bubble size was increased monotonously with time in this case. In moderate and high subcooling experiments, bubbles were detached from the wall after sliding for several millimeters and migrated towards the subcooled bulk liquid. The bubbles then reversed the direction of lateral migration and were reattached to the wall at moderate subcooling while they collapsed due to the condensation at high subcooling. It was hence considered that the mechanisms of the heat transfer from heated wall and the axial growth of vapor volume were influenced by the difference in bubble rise path. It was observed after the inception that bubbles were varied from flattened to more rounded shape. This observation suggested that the bubble detachment is mainly caused by the change in bubble shape due to the surface tension force.