Heat Transfer Mechanisms During Flow Boiling in Microchannels

Similarities and Differences Between Flow Boiling in Microchannels and Pool Boiling

The interrelations among different nondimensional groups shown by M. M. Awad are quite correct. The insight provided by the author is very much appreciated.The use of the correct nondimensional group is derived mainly from the physical interpretation of the underlying processes. Whenever possible, an underlying model based on force balance or some such similar approach is recommended. This will help in arriving at equations and correlations that are able to accurately represent the form of the interrelationships among variables. Such an approach was adopted by Kandlikar [(1)] in modeling the flow boiling CHF, resulting in the following equations from a force balance analysis at the liquid–vapor–solid contact line at the CHF condition.1FM'=a1(FS,1'+FS,2')+a2FI'+a3Fτ'In this equation, a1, a2, and a3 are constants, and F′ represents the forces per unit length of the contact line, and subscripts stand for the respective forces as follows:M—evaporation momentumS,1 and S,2—surface tension at the advancing contact line and the top of the bubble interface, respectivelyI—inertiaτ—viscousDividing by the surface tension forces throughout and simplifying results in the desired relationship among the nondimensional groups of interest in this discussion.2K2,CHF=a1(1+cosθR)+a2We(1-x)+a3Ca(1-x)where K2,CHF represents the nondimensional group K2 at the CHF condition, θR is the receding contact angle, and x is the vapor quality of the flow in the channel. As suggested by Awad, the Weber number may be replaced by the ratio K2/K1, but it may be better to retain the Weber number to clearly identify the contribution from the inertia term.In analyzing the liquid–vapor interface during pool and flow boiling, there are four forces that have been identified to be of interest. These are evaporation momentum, surface tension, inertia, and viscous forces. The following six combinations emerge as nondimensional groups.3Evaporation Momentum ForceInertia force=K14Evaporation Momentum ForceSurface tension force=K25Evaporation Momentum ForceViscous force=K36Inertia ForceSurface tension force=We,Weber number7Inertia ForceViscous Force=Re,Reynolds number8Viscous ForceSurface Tension Force=Ca,Capillary numberThe nondimensional group K3 has not been independently used yet, but it is relevant if the evaporation momentum and viscous forces are considered in a process. K3 can also be represented as:9K3=K1Re=K2/CaIn summary, recognizing the evaporation momentum force as an important force during the boiling process opens up the possibilities of three new relevant nondimensional groups, K1, K2, and K3. Any two of these groups can be represented by combining the third one with one of the other relevant nondimensional groups Re, We, and Ca.

High-fidelity simulation of flow boiling in microchannels remains a challenging problem, but the increasing interest in applications of microscale two-phase transport highlight its importance. In this paper, a volume of fluid (VOF)-based flow boiling model is proposed with computational expense-saving features that enable cost-effective simulation of two-phase flow and heat transfer in realistic geometries. The vapor and liquid phases are distinguished using a color function which represents the local volume fraction of the tracked phase. Mass conservation is satisfied by solving the transport equations for both phases with a finite-volume approach. In order to predict phase change at the liquid-vapor interface, evaporative heat and mass source terms are calculated using a novel, saturated-interface-volume phase change model that fixes the interface at the saturation temperature at each time step to achieve stability. Numerical oscillation of the evaporation source terms is thus eliminated and a non-iterative time advancement scheme can be adopted to reduce computational cost. The reference frame is set to move with the vapor slug to artificially increase the local velocity magnitude in the thin liquid film region in the relative frame, which reduces the influence of numerical errors resulting from calculation of the surface tension force, and thus suppresses the development of spurious currents. This allows use of non-uniform meshes that can efficiently resolve high-aspect-ratio geometries and flow features and significantly reduces the overall numerical expense. The proposed model is used to simulate the growth of a vapor bubble in a heated 2D axisymmetric microchannel. The bubble motion, bubble growth rate, liquid film thickness, and local heat transfer coefficient along the wall are compared against previous numerical studies.

An experimental investigation was performed to study the flow boiling heat transfer of CO2 in microchannels. Tests were conducted in a horizontal triangular microchannel with the hydraulic diameter of 0.86 mm. Heat to the test section was provided by direct electrical heating. Experiments were conducted with CO2 at saturation temperatures of 273 to 293 K, mass fluxes of 100 to 820 kg/m2s, heat fluxes of 3 to 23 kW/m2, and qualities of 20% to 85%. It was demonstrated that heat flux had an enhancing effect on the heat transfer coefficient, while mass flux had a negligible effect. Nucleate boiling mechanism is found to be the dominant factor for CO2 flow boiling in microchannels. Heat transfer coefficient degraded quickly at high vapor quality region (0.6–0.7), which is possibly due to flow mal-distribution. Pressure drop increases slightly with vapor quality and/or heat flux. Mass flux has a strong increasing effect on pressure drop.

A numerical study has been performed to analyze the wall heat transfer mechanisms during growth of multiple vapor bubbles inside a microchannel. The microchannel is of 200 μm square cross section and multiple vapor bubble begins to grow at once and after delay on 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. It can be seen that the second bubble which is far from the inlet always got bigger than the first one. When the second bubble nucleates 0.0549 ms after the first one, both bubbles grow almost to the same size. The numerical results show that bubbles with the lowest contact angle resulted in the highest wall heat transfer. Finally, a comparative study was done between single bubble and two bubbles inside a microchannel. wall heat transfer is better when two bubbles do not merge together compared to the single bubble and when they do merge.

Numerous studies have investigated how liquid water behaves on solid surfaces with uniformly hydrophilic or uniformly hydrophobic wetting properties. In particular, uniformly hydrophobic surfaces have been widely studied for modifying flow behavior of rivulets and drops at smaller scales, as well as for drag reduction on ships or other free-surface-piercing bodies at larger scales. Despite the extensive body of work on surfaces with uniform wetting properties, minimal work has been done to investigate how combining hydrophilic and hydrophobic regions onto a single surface to create macroscopic non-uniform wetting properties affects flows. Research in this vein has predominantly focused on low Reynolds number flows, such as in microfluidic channels or droplet impacts. This thesis expands on the current literature by investigating contact line dynamics and global flow behavior on surfaces with larger-scale non-uniform wetting properties. Experiments were first carried out to study thin sheet flow down an inclined plate at Re ~ 50 - 1200. The plate's wetting condition was changed by introducing alternating hydrophilic and hydrophobic bands 2-25 mm wide oriented at different angles with respect to the flow direction. Results show that the contact line of such flows is heavily modified compared to the uniform cases. At low Reynolds numbers, large-scale wettability heterogeneities are observed to tune the fingering instability wavelength if the bands are parallel to the flow direction and to dampen finger oscillations if the bands are perpendicular to the flow direction. At higher Reynolds numbers, roller structures are introduced at every hydrophilic-to-hydrophobic junction, modifying the global flow morphology. Entrained air bubbles are also captured and observed to coalesce if the bands are perpendicular to the flow direction. These experiments were then extended to a surface-piercing hydrofoil coated with alternating hydrophilic and hydrophobic bands. Experiments were run in Caltech's Free Surface Laboratory water tunnel for Re on the order of 104 to 105. The experiments demonstrate that the contact line is modulated in this context, alternating from concave to convex over the different wettability regions. The modulation of the contact line propagates to the rest of the water free-surface via the generation of standing waves and further modifies the free-surface separation point's location and steadiness. In addition, changes in wettability are observed to generate side force, which is of interest for vessel maneuvers in naval applications.

Experimental and Numerical Investigations of Heat Transfer During Flow Boiling in Microchannels

Review of Enhancement Techniques With Vapor Extraction During Flow Boiling in Microchannels

Two-phase flow of R-134a with high confinement number was experimentally carried out in this study. Flow boiling conditions for different orientations were controlled to take place in a stainless steel tube having a diameter of 0.5 mm. Based on a saturation pressure of 8 bar, a heat flux range of 2–26 kW/m2, and a mass flux range of 610–815 kg/m2s, a constant surface heat flux condition was controlled by applied DC power supply on the test section. The flow behaviors were described based on flow pattern and pressure drop data while heat transfer mechanisms were explained by using heat transfer coefficient data. In this work, nucleate boiling was observed, and the importance of the change in the flow direction was neglected, corresponding to the confinement number of around 1.7.

The current chapter presents the heat transfer and pressure data of three different dimension microgap heat sinks collected during the experimental program. Experimental results are discussed to characterize the two phase flow boiling heat transfer and pressure drop behaviors in microgap channel. Vapor confinement criteria was adopted from Harirchian and Garimella [1]. and experimental data was plotted based on this criterion to predict the vapor confinement inside the microgap channel under different operating conditions and microgap dimension. The heat transfer mechanisms are explained based on the vapor confined criteria and flow boiling regime. High speed visualizations are shown to validate the explanation.

Flow-pattern based heat transfer correlations for elongated bubbly flow and annular flow in stable flow boiling micro/minichannels were developed separately based on a collected micro/minichannel heat-transfer database by using an improved conventional-to-micro/minichannel threshold Bo = 4 and BoRel0.5 = 200, where Bo is the Bond number and Rel is the liquid Reynolds number. As significant disagreement in experimental trends and heat transfer mechanisms was reported for flow boiling in micro/minichannels in the literature, it is not possible to explain the discrepancy and predict all data points by a single correlation without considering the different flow patterns. The newly developed flow-pattern based predictive tool cannot only present a decent overall accuracy, but also estimate the parametric trends relatively well. Over 95% of the data points can be predicted by the proposed correlations within a ±50% error band for both elongated bubbly flow and annular flow. Therefore, the flow-pattern based correlations can be applied for heat exchanger design to improve the understanding of the underlying heat transfer mechanisms and to guide the development of further enhancement techniques for flow boiling in micro/minichannels.

Effect of Liquid-Vapor Phase Distribution on the Heat Transfer Mechanisms during Flow Boiling in Minichannels and Microchannels

Low Reynolds number flow through nozzle-diffuser elements in valveless micropumps

The experimental observation of flow patterns during flow boiling in microchannels has been an interesting field of work intended to enhance our understanding of the heat transfer and two-phase flow mechanisms. A number of studies have been conducted recently to observe the flow patterns during boiling in small channels with hydraulic diameters ranging from 200 μm to 1 mm. The present study is aimed at studying the flow patterns during flow boiling of water in two different channels of hydraulic diameter 207 μm and 333 μm respectively. Highspeed photography is used to obtain video images of the flow field. The channels are machined on top of a copper block and are covered with Lexan to permit visual observation. The copper block along with the channel is heated by a cartridge heater. The effect of channel height on flow patterns in single and multiple channels is studied as a function of mass flux, heat flux and location along the flow length. The results provide important information regarding the boiling behavior in different aspect ratio channels with single and mult iple parallel flow channels.

Voluminous research exists on the effect of combination of low Reynolds number flow over low aspect ratio flat plate wings for usage in micro-aerial vehicles and UAVs, as they also fly under similar conditions and combination of low aspect ratio and low Reynolds number. It has been well established by many researchers (Torres and Mueller in AIAA 42(5):865–873, 2004; Okamoto and Azuma in AIAA 49(6):1135–1150, 2011; Mizoguchi and Itoh in AIAA 51(7):7–52, 2013; Shields and Mohseni in AIAA 50(1):85–99, 2012; Mizoguchi and Kajikawa in Trans Jpn Soc Aero Sp Sci 59(2):56–63, 2016; Pelletier and Mueller in J Aircr 37(5):825–832, 2000; Liu and Hsiao in J Mech 28(01):77–89, 2012; Taira and Colonius in J Fluid Mech 623:187–207, 2009) that low aspect ratio wings in low Reynolds number flow gives a higher lift coefficient and stall angle is also increased which is quite different from the high aspect ratio wing. Also, it is a well-established fact in the existing literature that linear theories cannot be applied as in case of high aspect ratio wings because of non-linear increase in lift coefficient. The reason for such increase in lift and stall angle has been attributed to wingtip vortices that become prominent in low aspect ratio wings called the vortical lift which keeps on increasing up to a certain high angle of attack till it interacts with the separation bubble. In the past, very limited work has been done to study the vortices and their contribution in enhancement of aerodynamic characteristics. In the present work, flat plate model with corrugations at an aspect ratio (AR) of 1.0 are studied and compared with flat plate wing of AR 1.0; also flow visualization in water tunnel has been carried out at Reynolds number of 102 to ascertain the flow physics involved.