Hydrodynamic cavitation and boiling in refrigerant (R-123) flow inside microchannels

This research article investigates the effect that hydrodynamic cavitation has on heat transfer. The fluid medium is refrigerant R-123 flowing through 227 μm hydraulic diameter microchannels. The cavitation is instigated by the inlet orifice. Adiabatic tests were conducted to study the two-phase cavitating flow morphologies and hydrodynamic characteristics of the flow. Diabatic experiments were performed resulting in surface temperatures under heat fluxes up to 213 W/cm² and mass velocities from 622 kg/m² s to 1368 kg/m² s. Results were compared to non-cavitating flows at the same mass velocities. It was found that the cavitating flows can significantly enhance the heat transfer. The heat transfer coefficient of the cavitating flows was larger than the non-cavitating flows by as much as 84%. Single-phase and two-phase heat transfer coefficients have been elucidated and employed to deduce the heat transfer mechanism prevailing under boiling conditions with and without the presence of cavitation.     

Analysis of flow patterns emerging during evaporation in parallel microchannels

The evaporation processes of 2-propanol and water in cyclo olefin polymer (COP) and silicon microchannels of square cross-section are studied with a high-speed camera. The COP channels with a cross-section of 50 μm × 50 μm are rather smooth, whereas the 30 μm × 30 μm silicon channels have comparatively rough surfaces. For the COP channels, two different evaporation modes are identified, both with oscillating liquid–vapor menisci. One of these modes is characterized by an extremely rapid evaporation and a corresponding discontinuous shift of the meniscus. In the silicon channels four different evaporation modes are observed. Oscillatory motion of the liquid fronts also dominates here, and depending on the total mass flow and the wall temperature the oscillations in different channels are synchronized or desynchronized. Besides the flow patterns also the velocity trajectories of the evaporating liquid fronts are analyzed in detail and show a rather good reproducibility over different channels and different cycles. Compared to most other studies reported in this field, bubble nucleation is found to be of secondary importance for the evaporation processes.     

Visualization of flow patterns and bubble behavior during flow boiling in open microchannels

Flow boiling in microchannels is favored by the heat transfer community due to the high heat transfer rates that can be obtained with lower mass flow rates. However, the heat transfer rates for flow boiling in microchannels are impacted by flow reversals and flow instabilities. An open microchannel structure was recently proposed to reduce the impact of the flow boiling instabilities. Subcooled flow boiling experiments were conducted in open microchannels using deionized water. The open microchannels had 6 parallel channels with a 0.3 mm uniform thickness gap above them The channels were fabricated on a 6 mm × 40 mm copper block. The channels were 0.5 mm wide and 0.3 mm deep with 0.43 mm wide fins between them. Flow visualizations were performed with a high-speed CCD camera with the results showing that the flow regimes in the open microchannels differ from those in closed microchannels with stratified flow and no flow instability. Two types of confined bubbles were observed with characterizations of the effects of the bubbles on each other. The heat transfer mechanisms for flow boiling in open microchannels are also described.     

Non-linear analyses of flow boiling in microchannels

Recently, four unstable boiling cases with different fluctuating amplitudes were observed in parallel silicon microchannels having a hydraulic diameter of 186 μm. These were: the liquid/two-phase alternating flow (LTAF) at two different heat fluxes, the continuous two-phase flow (CTF) at medium heat flux and medium mass flux, and the liquid/two-phase/vapor alternating flow (LTVAF) at high heat flux and low mass flux. In this paper, data of these unstable boiling cases are analyzed using the following methods: correlation coefficient, attractor reconstruction, correlation dimension and largest Lyapunov exponent. The processes responsible for appearance of chaotic oscillations in microchannels, such as nucleation, stability of bubbly flow, vapour core stability and vapour-phase flow stability, are discussed. It is shown that under certain conditions, the microchannels system works as a thermal oscillator. It was found that heat supplied to the microchannels increases the heating surface temperature while the appearance of the two-phase flow inside the channels decreases the heating surface temperature. The mechanism involving an increase in heating surface temperature is supported by phenomena of blocking the liquid flow in microchannels by the two-phase flow.     

The effect of inlet constriction on bubble growth during flow boiling in microchannels

Flow boiling through microchannels is characterized by nucleation of vapor bubbles on the channel walls. In parallel microchannels connected through a common header, formation of vapor bubbles often results in flow mal-distribution that leads to reversed flow in certain channels. One way of eliminating the reversed flow is to incorporate flow restrictions at the channel inlet. In the present study, a nucleating vapor bubble placed near the restricted end of a single microchannel is numerically simulated. Placing restrictions at channel inlet increased the incoming liquid velocity for the same flow rate that prevented explosive bubble growth and reversed flow. It is proposed that channels with increasing cross-sectional area may be used to promote unidirectional growth of the vapor plugs and prevent reversed flow.     

Local measurement of flow boiling in structured surface microchannels

Experiments were conducted to investigate flow boiling in 200 μm × 253 μm parallel microchannels with structured reentrant cavities. Flow morphologies, boiling inceptions, heat transfer coefficients, and critical heat fluxes were obtained and studied for mass velocities ranging from G = 83 kg/m² s to G = 303 kg/m² s and heat fluxes up to 643 W/cm². Comparisons of the performance of the enhanced and plain-wall microchannels were performed. The microchannels with reentrant cavities were shown to promote nucleation of bubbles and to support significantly better reproducibility and uniformity of bubble generation. The structured surface was also shown to significantly reduce the boiling inception and to enhance the critical heat flux.     

Pressure effects on flow boiling instabilities in parallel microchannels

The effects of pressure on flow boiling instabilities in microchannels were experimentally studied. Experiments were conducted using water in 223 μm hydraulic diameter microchannels with mass fluxes ranging from 86 to 520 kg/m² s and pressures ranging from 50 to 205 kPa. Onset of flow oscillation, critical heat flux (CHF) conditions, local transient temperature measurements along with flow boiling visualization were obtained and studied. System pressure was found to significantly affect flow instabilities. For high pressure, it was observed that boiling instabilities were significantly delayed and CHF was extended to high mass qualities. Local temperature measurements also revealed lower magnitudes and higher frequencies of oscillations at high system pressures.     

Numerical investigation of subcooled flow boiling in segmented finned microchannels

Bubble growth behavior and heat transfer characteristics during subcooled flow boiling in segmented finned microchannels have been numerically investigated. Simulations have been performed for a single row of segmented finned microchannel and predicted results are compared with experimental investigations. Onset of nucleation, formation of bubbles, their growth and movements have been investigated for different values of applied heat flux. Mechanism of bubble expansion without clogging resulting in enhanced heat transfer in segmented finned microchannels has been explained. Temperature and pressure fluctuations during subcooled flow boiling condition have been investigated. It is observed that at high heat flux, thin liquid film trapped between the bubble and channel wall is evaporated leading to localized heating effect. Predicted flow patterns are similar to experimental results. However, simulations over predict the bubble growth rate and heat transfer coefficient.     

Capillary-assisted flow and evaporation inside circumferential rectangular micro groove

To introduce capillary-assisted evaporation from micro-size fields to normal-size fields, an inclined circumferential micro groove with rectangular cross sections is investigated analytically and a systematic mathematical model is developed. The model is composed of five sub-models: a natural convection model, a liquid axial flow model, a heat transfer model in and below the intrinsic meniscus, an evaporation thin film region model and an adsorbed region model. In this model, for the extended meniscuses formed at groove cross sections, both the intrinsic meniscus and evaporation thin film region are considered when calculating heat absorbing. Through solving the model, the influences of dynamic contact angle on the heat absorbing in the intrinsic meniscus and evaporation thin film region are investigated. Moreover, the factors affecting the whole-groove equivalent heat transfer coefficient have been investigated.     

Effects of inlet/outlet configurations on flow boiling instability in parallel microchannels

A simultaneous visualization and measurement study has been carried out to investigate effects of inlet/outlet configurations on flow boiling instabilities in parallel microchannels, having a length of 30 mm and a hydraulic diameter of 186 μm. Three types of inlet/outlet configurations were investigated. Fluid flow entering to and exiting from the microchannels with the Type-A connection was restricted because the inlet and outlet conduits were perpendicular to the microchannels. The fluid flow had no restriction in entering to and existing from the microchannels with the Type-B connection. In the Type-C connection, fluid flow was restricted in entering each microchannel but was not restricted in exiting from the microchannels. It is found that amplitudes of temperature and pressure oscillations in the Type-B connection are much smaller than those in the Type-A connection under the same heat flux and mass flux conditions. On the other hand, nearly steady flow boiling exists in the parallel microchannels with the Type-C connection under the experimental conditions. Therefore, this configuration is recommended for high-heat-flux microchannel applications. As predicted, the stability threshold is determined by the minimum in the pressure-drop-versus-flow-rate curve. The pressure drop and heat transfer coefficient versus vapor quality for flow boiling in microchannels with the Type-C connection are presented. It is found that experimental data of pressure drop are higher and heat transfer coefficients are lower for boiling flow at high vapor quality in microchannels than those predicted from correlation equations for boiling flow in macrochannels, due to local dryout.     

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 in microchannels and microgravity

A critical review of the state of the art of research on internal forced convection boiling in microchannels and in microgravity conditions is the main object of the present paper.     

Scale effects on flow boiling heat transfer in microchannels: A fundamental perspective

Flow boiling in microchannels has received considerable attention from researchers worldwide in the last decade. A scaling analysis is presented to identify the relative effects of different forces on the boiling process at microscale. Based on this scaling analysis, the flow pattern transitions and stability for flow boiling of water and FC-77 are evaluated. From the insight gained through the careful visualization and thermal measurements by previous investigators, similarities between heat transfer around a nucleating bubble in pool boiling and in the elongated bubble/slug flow pattern in flow boiling are brought out. The roles of microlayer evaporation and transient conduction/microconvection are discussed. Furthermore, it is pointed out that the convective contribution cannot be ruled out on the basis of experimental data which shows no dependence of heat transfer coefficient on mass flow rate, since the low liquid flow rate during flow boiling in microchannels at low qualities leads to laminar flow, where heat transfer coefficient is essentially independent of the mass flow rate. Specific suggestions for future research to enhance the boiling heat transfer in microchannels are also provided.     

Unstable and stable flow boiling in parallel microchannels and in a single microchannel

A simultaneous visualization and measurement study have been carried out to investigate flow boiling instabilities of water in microchannels at various heat fluxes and mass fluxes. Two separate flow boiling experiments were conducted in eight parallel silicon microchannels (with flow interaction from neighboring channels at headers) and in a single microchannel (without flow interaction), respectively. These microchannels, at a length of 30 mm, had an identical trapezoidal cross-section with a hydraulic diameter of 186 μm. At a given heat flux and inlet water temperature, it was found that stable and unstable flow boiling regimes existed, depending on the mass flux. A flow boiling map, in terms of heat flux vs mass flux, showing stable flow boiling regime and unstable flow boiling regime is presented for parallel microchannels as well as for a single microchannel, respectively, at an inlet water temperature of 35 °C. In the stable flow boiling regime, isolated bubbles were generated and were pushed away by the incoming subcooled liquid. Two unstable flow boiling regimes, with long-period oscillation (more than 1 s) and short-period oscillation (less than 0.1 s) in temperature and pressure, were identified. The former was due to the expansion of vapor bubble from downstream while the latter was owing to the flow pattern transition from annular to mist flow. A comparison of results of flow boiling in parallel microchannels and in a single microchannel shows that flow interaction effects from neighboring channels at the headers are significant.     

Effect of aspect ratio on saturated boiling flow in microchannels with nonuniform heat flux

Microchannel two‐phase flow is an effective cooling method used in microelectronics, in which the heat flux density is unevenly distributed usually. The paper is focused on numerical study the effect of aspect ratio on the flow boiling of microchannels with nonuniform heat flux. The heat source is a three‐dimensional (3D) integrated circuit. 3D microchannel model and volume of fluid method are coupled in numerical simulation. The results show that the aspect ratio has no relationship with the two‐phase pressure drop of the microchannel. It has a certain influence on the distribution of bubble shape. In terms of the heat transfer coefficient, the aspect ratio has a certain influence on a section of the inlet. Due to the nucleate boiling, the convective heat transfer in the remaining areas is the dominant factor and the average heat transfer coefficient is mainly determined by the heat flux at the bottom of the channel.     

Jet flow phenomena during nucleate boiling

Boiling phenomena are with highly complex nonlinear and nonequilibrium characteristics, which cause diversity and complexity of boiling nucleation. In the present paper, an experimental investigation was conducted to investigate the nucleate boiling behavior on a very fine heating wire. Using zoom routine and CCD camera system, the dynamical process of nucleate boiling was visually observed and several modes of jet flows were explored during nucleate boiling. This phenomenon is quite different from the usual observation of nucleate boiling. High-energy liquid jet, fog-like jet, cluster-like jet, bubble-forming jet, bubble-bunch jet and bubble-top jet were described in detail. The microscopic mechanism concerning the phenomena was discussed.     

On the eruptive boiling in silicon-based microchannels

This study investigates experimentally eruptive boiling in a silicon-based rectangular microchannel with a hydraulic diameter of 33.7 μm, a width of 99.8 μm and a depth-to-width ratio of 0.203. The microchannel is made of SOI wafer and prepared using bulk micro-machining and anodic bonding. The surface roughness for both the bottom and the side walls was measured using an atomic force microscope. The evolution of the eruptive boiling of water in the smooth microchannel was clearly observed using an ultra high-speed video camera (up to 50,000 frames/s) at mass fluxes of 417 and 625 kg/m² s and a heat flux from 14.9 to 372 kW/m². It is confirmed that eruptive boiling is a form of rapid bubble nucleation after which the bubble merges with a slug bubble downstream in a short distance or evolve to a slug bubble. The bubble frequency in all of the cases studied is provided. Eruptive boiling may be predicted classically with nano-sized cavities that are consistent with the measured surface roughness.     

Refrigerant flow boiling heat transfer in parallel microchannels as a function of local vapor quality

Flow boiling of refrigerant HFC-134a in a multi-microchannel copper cold plate evaporator is investigated. The heat transfer coefficient is measured locally for the entire range of vapor qualities starting from subcooled liquid to superheated vapor. The test piece contains 17 parallel, rectangular microchannels (0.762 mm wide) of hydraulic diameter 1.09 mm and aspect ratio 2.5. The design of the test facility is validated by a robust energy balance as well as a comparison of single-phase heat transfer coefficients with results from the literature. Results are presented for four different mass fluxes of 20.3, 40.5, 60.8, and 81.0 kg m^?2 s^?1, which correspond to refrigerant mass flow rates of 0.5–2.0 g s^?1, and at three different pressures 400, 550 and 750 kPa corresponding to saturation temperatures of 8.9, 18.7, and 29 °C. The wall heat flux varies from 0 to 20 W/cm² in the experiments. The heat transfer coefficient is found to vary significantly with refrigerant inlet quality and mass flow rate, but only slightly with saturation pressure for the range of values investigated. The peak heat transfer coefficient is observed for a vapor quality of approximately 20%.     

Numerical modelling of boiling heat transfer in microchannels

In this paper we report the results of our modelling studies on two-phase forced convection in microchannels using water as the fluid medium. The study incorporates the effects of fluid flow rate, power input and channel geometry on the flow resistance and heat transfer from these microchannels. Two separate numerical models have been developed assuming homogeneous and annular flow boiling. Traditional assumptions like negligible single-phase pressure drop or fixed inlet pressure have been relaxed in the models making analysis more complex. The governing equations have been solved from the grass-root level to predict the boiling front, pressure drop and thermal resistance as functions of exit pressure and heat input. The results of both the models are compared to each other and with available experimental data. It is seen that the annular flow model typically predicts higher pressure drop compared to the homogeneous model. Finally, the model has also been extended to study the effects of non-uniform heat input along the flow direction. The results show that the non-uniform power map can have a very strong effect on the overall fluid dynamics and heat transfer.     

Interfacial heat transfer during subcooled flow boiling

In order to develop a mechanistic model for the subcooled flow boiling process, the key issues which must be addressed are wall heat flux partitioning and interfacial (condensation) heat transfer. The sink term in the two-fluid models for void fraction prediction is provided by the condensation rate at the vapor-liquid interface. Low pressure subcooled flow boiling experiments, using water, were performed using a vertical flat plate heater to investigate the bubble collapse process. A high-speed CCD camera was used to record the bubble collapse in the bulk subcooled liquid. Based on the analyses of these digitized images, bubble collapse rates and the associated heat transfer rate were determined. The experimental data were in turn used to correlate the bubble collapse rate and the interfacial heat transfer rate. These correlations are functions of bubble Reynolds number, liquid Prandtl number, Jacob number, and Fourier number. The correlations account for both the effect of forced convection heat transfer and thickening of the thermal boundary layer as the vapor bubble condenses which in turn makes the condensation heat transfer time dependent. Comparison of the measured experimental data with those predicted from the correlations show that predictions are well within ±25% of the experimentally measured values. These correlations have also been compared with those available in the literature.