Unsteady-state fluctuations analysis during bubble growth in a “rectangular” microchannel

Boiling in microchannels shows great potential for cooling systems and compact heat removal applications. However for confidence in this cooling technique, it is essential that any excursions from typical flow boiling are understood and predicted. Confined bubble growth can cause pressure fluctuations which interfere with bubble nucleation and growth and can also lead to flow reversal and instances of temperature excursions. Boiling experiments are performed in a single rectangular microchannel of hydraulic diameter 771 μm, using n-Pentane as the working fluid. A heating technique was incorporated on the exterior walls of the microchannel; a transparent, metallic, conductive deposit, which allows simultaneous uniform heating and visualisation to be achieved. In conjunction with obtaining high-speed imaging, an infrared camera is used to record the temperature profile at the microchannel wall, and sensitive pressure sensors are used to record the pressure drop across the microchannel over time. During flow boiling in the microchannel periodic and non-periodic fluctuations in both the channel pressure drop and channel temperature profile over time are apparent. In this paper we provide a full analysis of the temperature measurements and pressure data obtained during the growth of a vapour bubble in the microchannel. An augmentation of the heat transfer coefficient of over 216% has been achieved during periodic two-phase flow boiling in the microchannel. However overpressure (over 410% increase) in the microchannel occurs at corresponding instances to the heat transfer enhancement. The two time steps during the periodic bubble dynamics, namely the bubble expansion time period and the waiting time period in-between the bubble expansion fluctuations, are also investigated and modelled. It was determined that both the bubble dynamics and the channel wall heating time period are responsible for the pressure and temperature fluctuation time periods observed.     

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.     

Physical mechanisms of heat transfer during single bubble nucleate boiling of FC-72 under saturation conditions. II: Theoretical analysis

This paper is the second part of a two-part study concerning the dynamics of heat transfer during the nucleation process of FC-72 liquid. The experimental findings on the nature of different heat transfer mechanisms involved in the nucleation process were discussed in part I. In this paper, the experimental results are compared with the existing boiling models. The boiling models based on dominance of a single mechanism of heat transfer did not match the experimental results. However, the Rohsenow model was found to closely predict the heat transfer through the microconvection mechanism that is primarily active outside the bubble/surface contact area. An existing transient conduction model was modified to predict the surface heat transfer during the rewetting process (i.e. transient conduction mechanism). This model takes into account the gradual rewetting of the surface during the transient conduction process rather than a simple sudden surface coverage assumption commonly used in the boiling literature. The initial superheat energy of the microlayer (i.e. microlayer sensible energy) was accurately calculated and found to significantly contribute in microlayer evaporation. This even exceeded the direct wall heat transfer to microlayer at high surface superheat temperatures. A composite model was introduced that closely matches our experimental results. It incorporates models for three mechanisms of heat transfer including microlayer evaporation, transient conduction, microconvection, as well as their influence area and activation time. The significance of this development is that, for the first time, all submodels of the composite correlation were independently verified using experimental results.     

Microscale heterogeneous boiling on smooth surfaces—from bubble nucleation to bubble dynamics

In this investigation, boiling incipience and bubble dynamics on a microheater with a geometry of 100 μm × 100 μm fabricated with MEMS technology are evaluated using a high-speed digital camera. For the purpose of comparison with conventional boiling heat transfer, boiling incipience and bubble dynamics are also studied on a carefully selected microheater with a fabricated defect (i.e., a microcavity on the heater surface). Of industrial interest are the effects of dissolved gases on boiling incipience and bubble dynamics, which are also discussed in detail. The possible nucleation temperature (or incipience temperature) is analyzed and discussed from the perspective of the measured bulk temperature of the microheater and a 3D heat conduction numerical model. The time-resolved bubble dynamics (i.e., the bubble size evolution, interface velocity and interface acceleration) are all presented along with high-speed digital images. Based upon this investigation, it is clear that explosive boiling can take place on a smooth surface no matter how slow the heating rate, and dissolved gases have a significant influence on the incipience temperature and bubble behavior. Furthermore, this study illustrates that the classical kinetics of boiling can explain the explosive boiling occurring on a smooth surface in principle and can provide a useful guide for the design of microscale heat transfer and/or MEMS devices. Although unexpected, due to the gravitational effects, Marangoni flow on the vapor–liquid interface induced by the temperature gradient was also observed.     

Saturated flow boiling heat transfer of R-407C and associated bubble characteristics in a narrow annular duct

Experiments are conducted here to investigate how the channel size affects the R-407C saturated flow boiling heat transfer and associated bubble characteristics in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. The measured data indicate that the saturated flow boiling heat transfer coefficient increases with a decrease in the duct gap. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are milder. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases noticeably at increasing R-407C mass flux. Moreover, the bubble departure frequency increases at reducing duct size and at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Meanwhile comparison of the present heat transfer data for R-407C with R-134a in the same duct and with some existing correlations is conducted. Furthermore, correlation for the present R-407C saturated flow boiling heat transfer data is proposed. Additionally, the present data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated.     

Study of bubble growth in water pool boiling through synchronized,infrared thermometry and high-speed video

High-speed video and infrared thermometry were used to obtain time- and space-resolved information on bubble nucleation and heat transfer in pool boiling of water. The bubble departure diameter and frequency, growth and wait times, and nucleation site density were directly measured for a thin, electrically-heated, indium–tin-oxide surface, laid on a sapphire substrate. These data are very valuable for validation of two-phase flow and heat transfer models, including computational fluid dynamics with interface tracking methods. Here, detailed experimental bubble-growth data from individual nucleation sites were used to evaluate simple, commonly-used, but poorly-validated, bubble-growth and nucleate-boiling heat-transfer models. The agreement between the data and the models was found to be reasonably good. Also, the heat flux partitioning model, to which our data on nucleation site density, bubble departure diameter and frequency were directly fed, suggests that transient conduction following bubble departure is the dominant contribution to nucleate-boiling heat transfer.     

Subcooled flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct

Experiments are conducted here to investigate how the channel size affects the subcooled flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. From the measured boiling curves, the temperature undershoot at ONB is found to be relatively significant for the subcooled flow boiling of R-134a in the duct. The R-134a subcooled flow boiling heat transfer coefficient increases with a reduction in the gap size, but decreases with an increase in the inlet liquid subcooling. Besides, raising the imposed heat flux can cause a substantial increase in the subcooled boiling heat transfer coefficient. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are small in the narrow duct. Visualization of the subcooled flow boiling processes reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Moreover, raising the imposed heat flux significantly increases the bubble population, coalescence and departure frequency. The increase in the bubble departure frequency by reducing the duct size is due to the rising wall shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities on the heating surface tend to merge together to form big bubbles. Correlation for the present subcooled flow boiling heat transfer data of R-134a in the narrow annular duct is proposed. Additionally, the present data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated.     

Boiling heat transfer in vertical minichannels. Liquid crystal experiments and numerical investigations

The paper presents the results of experimental and numerical studies of boiling heat transfer in the flow of refrigerants R123 and R11 through vertical, rectangular minichannels, with one wall heated. An application of liquid crystal thermography has helped detect two-dimensional temperature distribution on the heating surface, allowing determination of boiling heat fluxes and experimental boiling curves. The main objectives of the paper included the development of two-dimensional approach to solve the inverse heat conduction boundary problem for determining local values of internal heating surface temperature, boiling heat flux and heat transfer coefficient, and the improvement of the applied numerical method making use of the equalizing calculus and heating surface temperature measurement errors. A detailed discussion of temperature, heat flux and heat transfer coefficient errors is also provided.     

Saturated flow boiling heat transfer of refrigerant R-410A in a horizontal annular finned duct

An experiment is conducted here to investigate the saturated flow boiling heat transfer characteristics of ozone friendly refrigerant R-410A in a horizontal annular finned duct. Meanwhile the associated bubble characteristics in the duct are also inspected from the flow visualization. The experimental data are presented in terms of saturated flow boiling curves, boiling heat transfer coefficients and flow photos. In addition, empirical correlation equations for the saturated flow boiling heat transfer coefficient and mean bubble departure diameter are proposed. The saturated flow boiling curves show that boiling hysteresis is insignificant in the flow and the wall superheat needed for the onset of nucleate boiling is slightly affected by the refrigerant mass flux. Besides, the boiling curves are mainly affected by the imposed heat flux and refrigerant mass flux. Moreover, the measured saturated flow boiling heat transfer coefficient increases with the imposed heat flux and refrigerant mass flux. Furthermore, at a higher refrigerant mass flux the departing bubbles are smaller.     

A Study of Bubble Behavior and Boiling Heat Transfer Enhancement under Electric Field

The effect of a D.C. electric field on nucleate boiling heat transfer for refrigerants, R11, R113, and FC72, was investigated experimentally in a single-tube shell/tube heat exchanger by using the temperature control method of wall superheat. Also the behavior of bubble under nonuniform electric field produced by wire electrodes was studied by numerical calculation. For R11, the electrohydrodynamic (EHD) enhancement for boiling heat transfer was observed for all ranges of wall superheat tested. However, the enhancement in boiling heat transfer disappeared if the wall superheat exceeded 13°C for R113, and no electric field effect on the boiling heat transfer was observed for FC72. An application of approximately 5 kV was enough to eliminate the boiling hysteresis for R11 and R113. Numerical study of the electric field in a single medium has hinted that the bubbles are forced away from the heating surface and toward the electrostatic stagnation point by the dielectrophoretic force. Such modified bubble motion turns out to promote the boiling heat transfer if one uses proper electrode configuration.     

Numerical simulation of nucleate boiling on a horizontal surface at high heat fluxes

Nucleate boiling at high heat fluxes has been studied numerically by solving the equations governing conservation of mass, momentum and energy in the liquid and vapor phases. The interface is captured by using the level set method based on a sharp-interface representation. The evaporative heat flux from the liquid microlayer is incorporated in the analysis. The effects of wall superheat, number density of nucleation sites and waiting period on the bubble dynamics and heat transfer in nucleate boiling are investigated. The heat fluxes obtained from the present numerical simulations are compared with the experimental data reported in the literature.     

Numerical Simulation of Bubbles Deformation,Flow, and Coalescence in a Microchannel Under Pseudo-Nucleation Conditions

This paper reports the results of numerical study on bubbles deformation, flow, and coalescence under pseudo-nucleate boiling conditions in horizontal mini-/microchannels. The numerical simulation, which is based on the multiphase model of volume of fluid method, aims to study the corresponding flow behaviors of nucleate bubbles generated from the tube walls in mini-/microchannels so as to understand the effect of confined surfaces/walls on nucleate bubbles and heat transfer. Under the pseudo- or quasi-nucleate boiling condition, superheated small vapor bubbles are injected at the wall to ensure that the bubbles generation is under a similar condition of real nucleation. The numerical study examined the fluid mechanics of bubble motion with heat transfer, but the mass transfer across the bubble–liquid interface is not simulated in the present work.     

Numerical study of bubble growth and wall heat transfer during flow boiling in a microchannel

A numerical study has been performed to analyze the wall heat transfer mechanisms during growth of a vapor bubble inside a microchannel. The microchannel is of 200 μm square cross section and a vapor bubble begins to grow at one of the walls, with liquid coming in through the channel inlet. The complete Navier–Stokes equations along with continuity and energy equations are solved using the SIMPLER method. The liquid vapor interface is captured using the level set technique. Experiments have been conducted to validate the numerical model. The bubble growth rate and shapes show good agreement between numerical and experimental results. The numerical results show that the wall heat transfer increases with wall superheat but stays almost unaffected by the liquid flow rate. The liquid vapor surface tension value has little influence on bubble growth and wall heat transfer. However, the bubble with the lowest contact angle resulted in the highest wall heat transfer.     

Nucleate boiling in two types of vertical narrow channels

To explore the mechanism of boiling bubble dynamics in narrow channels, we investigate 2-mm wide I- and Z-shaped channels. The influence of wall contact angle on bubble generation and growth is studied using numerical simulation. The relationships between different channel shapes and the pressure drop are also examined, taking into account the effects of gravity, surface tension, and wall adhesion. The wall contact angle imposes considerable influence over the morphology of bubbles. The smaller the wall contact angle, the rounder the bubbles, and the less time the bubbles take to depart from the wall. Otherwise, the bubbles experience more difficulty in departure. Variations in the contact angle also affect the heat transfer coefficient. The greater the wall contact angle, the larger the bubble-covered area. Therefore, wall thermal resistance increases, bubble nucleation is suppressed, and the heat transfer coefficient is lowered. The role of surface tension in boiling heat transfer is considerably more important than that of gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of the I-shaped type, and the pressure drop of the former is clearly higher.     

Modeling and numerical investigation of hydrogen nucleate flow boiling heat transfer

Liquid hydrogen flow boiling heat transfer in tubes is of great importance in the hydrogen applications such as superconductor cooling, hydrogen fueling. In the present study, a numerical model for hydrogen nucleate flow boiling based on the wall partition heat flux model is established. The key parameters in the model such as active nucleation site density, bubble departure diameter and frequency are carefully discussed and determined to facilitate the modeling and simulation of hydrogen flow boiling. Simulation results of the numerical model show reasonably well agreement with experimental data from different research groups in a wide operation condition range with the means absolute error (MAE) of 10.6% for saturated and 5.3% for subcooled flow boiling. Based on the model, wall heat flux components and void fraction distribution of hydrogen flow boiling are studied. Effects of mass flow rate and wall heat flux on the flow boiling heat transfer performance are investigated. It is found that in the hydrogen nucleate flow boiling, the predominated factor is the Boiling number, rather than the vapor quality. A new simple correlation is proposed for predicting hydrogen saturated nucleate flow boiling Nusselt number. The MAE between the correlation predicted and experimentally measured Nusselt number is 13.6% for circular tubes and 12.5% for rectangular tubes. The new correlation is applicable in the range of channel diameter 4–6.35 mm, Reynolds number 64000–660,000, saturation temperature 22–29 K, Boiling number 8.37 × 10^?5–2.33 × 10^?3.     

Saturated flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct

Experiments are conducted here to investigate how the channel size affects the saturated flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. The measured heat transfer data indicate that the saturated flow boiling heat transfer coefficient increases with a decrease in the gap of the duct. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are milder. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases slightly at increasing R-134a mass flux. Moreover, the bubble departure frequency increases at reducing duct size mainly due to the rising shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Correlation for the present saturated flow boiling heat transfer data of R-134a in the narrow annular duct is proposed. Additionally, data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated.     

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.     

Unsteady Natural Convective Boiling in a Narrow Vertical Channel

The purpose of this work is to describe the two-phase flow structure and heat transfer of unsteady natural convective boiling in a narrow vertical channel. The experiments are performed with saturated n-pentane at a pressure of 1 bar. An unheated plate is placed parallel to the heating surface and lateral sides are closed. The distance between the heated surface and the confinement plate is 800 μm. Void fraction measurements are performed using capacitive sensors. The void fraction increases with heat flux and reaches a maximum of 0.80 in the mid-height of the channel when the heat flux is equal to 90% of the critical heat flux. Flow observations using a high-speed video camera show an unsteady thermo-hydraulic behavior. The frequency of the cycles increases with the wall temperature during nucleate and transient boiling. Local velocities of the bubble meniscus developing within the confined space are determined during the boiling cycles. The time-averaged liquid flow rate increases significantly with heat flux and reaches a maximum for heat flux close to the critical heat flux.     

Numerical study of patterns and influencing factors on flow boiling in vertical tubes by thermal LBM simulation

Heat transfer and flow pattern of flow boiling in vertical tube are investigated numerically based on the phase-change Lattice Boltzmann method (LBM) which includes an improved pseudo-potential LB model and a thermal LB model. Two-dimensional numerical simulations are carried out under constant heat flux conditions for the first time. The processes of growth, slippage, detachment and coalescence of the bubbles are captured to verify the correctness of the model. The effects of gravity, contact angle and wall superheat on bubble departure diameter and nucleation waiting time are illustrated. The multiple flow patterns, single-phase flow, bubble flow, slug flow and DNB have been illustrated with the behaviors of bubble nucleation, growth, departure, and coalescence. Some basic features of flow boiling have been clearly observed in the simulation. The influences of several factors such as heat flux, Reynolds number, the width of flow channel, and the width of nucleation point on flow boiling especially on the point of DNB are investigated. The numerical results show that the DNB could be avoided by reducing the heating density, increasing the Reynolds number, increasing the width of the tube and reducing the heating concentration.     

Hydrodynamics and heat transfer prior to onset of nucleate boiling in a rectangular microchannel heat sink

The conjugate heat transfer of flow boiling in a rectangular microchannel heat sink (MCHS) was modelled numerically to investigate the hydrodynamics and thermal responses of flow prior to the onset of nucleate boiling (ONB). Local hydrodynamics and thermal conditions leading to ONB are analysed numerically for different heat flux. The flow patterns of different modes of microconvection and mixed convective flows including the circulating flow, wavy flow and seeping flow were demonstrated and discussed. The numerical study proposes the mechanism leading to the first bubble nucleation which cover the initiation of fluid instability until the ONB. This work provides better understanding of the superheat induced flow instability and the progressive fluid convection under transient heating.