Evaporation Of Thin Liquid Film Research Articles

Flow pattern-based heat transfer analysis of microchannel flow boiling: An experimental investigation

The present study investigates the flow boiling heat transfer and flow pattern of R245fa in a smooth, horizontal microchannel with an inner diameter of 1 mm. The experiments cover a range of mass flux from 300 to 700 kg/m2s and heat fluxes varying between 10 and 94 kW/m2, with corresponding saturation temperature of 23–54 °C. Heat transfer experiment results indicate that the local heat transfer coefficient (HTC) presents two distinct variation trends under different heat and mass flux conditions. In addition, the heat flux is positively correlated with the local HTC at each measure point. Visualization experiment results show that annular flow forms at low vapor quality and significantly influences heat transfer as the primary flow pattern. The flow boiling heat transfer of R245fa in the 1 mm microchannel can be characterized by three regimes: bubble/slug regime, liquid film evaporation regime, and intermittent dryout regime. Thin liquid film evaporation and intermittent dryout during annular flow predominately contribute to heat transfer. Finally, the experimental HTC is compared with five different predictive methods, revealing that the three-zone model proposed by Thome et al. [46] holds immense potential for estimating the local HTC in the microchannel.

Effects of cover plate wettability on pool boiling heat transfer in a narrow gap

An experimental study is conducted to evaluate the effects of cover plate wettability (hydrophobic to hydrophilic) on boiling heat transfer in a narrow horizontal gap (0.64mm to 5mm thick). When the gap thickness is greater than 1mm, the cover plate wettability plays a significant role. A hydrophobic cover plate causes vapor bubbles to move from the heating surface and get attached to the cover plate, maintaining liquid water on the heating surface. When the vapor layer does not reach the heating surface, the nucleate boiling heat transfer coefficient is hardly affected by the gap. However, the critical heat flux (CHF) is reduced relative to the unconfined case as vapor fills the narrow gap. Contrarily, when a hydrophilic cover plate is used with a gap thickness greater than 1mm, a vapor slug forms and hovers over the heating surface with transient lateral movement. Thin liquid film evaporation occurs under the slug, causing an enhanced heat transfer coefficient at low heat flux (q'' < 250kW/m2). As the heat flux increases, the vapor slug movement causes fluctuations in the heater temperature due to partial dryout at the heating surface. Consequently, a rapid decrease in the heat transfer and a reduced CHF are observed relative to the unconfined case. When the gap thickness is reduced to below 1mm, a film boiling-like behavior is observed regardless of the cover plate wettability. However, there is an increase in the heat transfer coefficient at low heat flux (q'' < 250kW/m2).

Molecular dynamics simulation of the evaporation of thin liquid sodium film on the conical nanostructure surface

Deeply understanding the evaporation of the nanoscale thin liquid film on the nanostructure substrates is significant for further constructing the novel wick structure with nanostructure surfaces, which would be used in the optimal design of the sodium heat pipe. For this purpose, this paper investigates the evaporation of thin liquid film on conical nanostructured surfaces by the molecular dynamics method. A cuboid evaporation system is constructed. It consists of the bottom solid substrate and thin liquid sodium film. A flat solid wall is set as the reference. By changing the nanoscale cone's number and height, five conical nanostructure surfaces are built. The number of gas atoms and the total energy of fluid atoms are observed respectively. The evaporation rate and the average heat flux (q) of six cases are compared. The conical nanostructure suppresses evaporation. With the roughness of the conical nanostructures, the evaporation rate decrement maximum can reach 83% and the q decrement maximum can reach 58%. Based on the thermal resistances in the solid-liquid contact region, the D (Self-diffusion coefficient), and the peak values of RDF (Radial distribution function), the mechanisms of the conical nanostructure surfaces are analyzed. The suppressing effects of the conical nanostructure surfaces are achieved by weakening the heat conduction in the solid-liquid contact region and mitigating the collision heat transfer of liquid atoms.

Boiling heat transfer of binary and ternary mixtures in multiple rectangular microchannels

In this study, the boiling heat transfer of binary and ternary refrigerant mixtures in a horizontal multiport tube with multiple rectangular microchannels was investigated. The influences of the mass velocity, vapor quality, and heat flux on the heat transfer were evaluated for near- and non-azeotropic mixtures of R410A, R32/CF3I, R32/R1123, R32/R1234yf, R1123/R32/R1234yf, and CO2/R32/R1234yf. The obtained results were compared with those of pure refrigerants R1234yf and R32. For R1234yf at low mass velocity, the flow patterns were estimated as intermittent (plug) and slug-annular flows with an extremely thin liquid film. The heat transfer coefficient decreased when the heat flux was increased because dry patch areas formed and expanded on the sides of the noncircular microchannel. The near-azeotropic mixtures with small temperature glides exhibited heat transfer characteristics similar to those of the pure refrigerant. Conversely, the heat transfer coefficients deteriorated slightly under lower mass velocities. The heat transfer coefficients of binary and ternary mixtures with larger temperature glides were significantly different from those of pure refrigerants and near-azeotropic mixtures; they decreased with decreasing mass velocity owing to mass diffusion resistance. The heat transfer coefficients of binary mixtures with large temperature glides decreased monotonically as the mass velocity decreased, and the heat transfer noticeably deteriorated at lower mass velocities. The heat transfer model was developed for rectangular microchannels by considering the heat transfer contributions of thin liquid film evaporation due to surface force, forced convection, and nucleate boiling. Moreover, this model considers the flow pattern of multiple microchannels, heat transfer degradations due to dry patches and mass diffusion resistance, and dryout incipience quality. The results of the present model agree well with the heat transfer coefficients, including those of binary and ternary mixtures, in the pre- and post-dryout regions with a mean percentage error of 3.1%.

Experimental estimation of the local heat transfer coefficient for thin liquid film evaporation in a capillary tube

• Local heat transfer coefficient was estimated for thin liquid film evaporation in a capillary tube. • Temperature infrared measurements were employed as input data of the inverse heat conduction problem. • A robust estimation procedure was adopted. • Procedure validation through the application to both synthetic and experimental data. • Original insight of thin liquid film evaporation was given. Two-phase heat transfer devices consisting of mini and microtubes represent a powerful solution to the continuous reduction of electronic components' dimensions and to the increase of the power to dissipate. Despite many studies on the two-phase heat transfer phenomenon in minitubes, most provided outcomes regarding the average heat transfer coefficient or Nusselt number. A technique to assess the local convective wall heat flux and heat transfer coefficient is proposed, validated, and then applied to the experimental case of thin-film evaporation in a heated capillary tube. The evaporation of the thin film of liquid plays a vital part in the two-phase heat transfer phenomenon inside mini/microtubes. Our study investigated this phenomenon by examining a semi-infinite slug flow. The estimation method used is represented by the inverse heat conduction problem solution in the tube wall, applying the Tikhonov regularisation method. The temperature distributions on the outer surface of the channel measured by an infrared camera were employed as input data for the inverse problem. The estimated heat transfer coefficient distributions confirm the available correlations in average values and provide insight into these complex phenomena in local and instantaneous quantities.

Bubble behaviors and flow boiling characteristics in open microchannels with large aspect ratio

Large aspect ratio of microchannels enables the corresponding heat sink possess higher heat transfer capability, however, the generated bubbles tend to rapidly extend forward and backward due to the large aspect ratio of the channels, thereafter blocking the channels, deteriorating the heat dissipation and increasing the pressure loss. Open microchannels have been reported to inhibit the two-phase flow instability due to the existence of extra gap for vapor expansion. However, the visualization of bubble dynamics and characterization of flow boiling behaviors of open microchannels with large aspect ratio are limited. In this study, the open microchannel heat sink with large aspect ratio was proposed and flow boiling tests were conducted at two mass fluxes of 153 and 229 kg/m2·s with the effective heat flux ranging of 180.81–2165.15 kW/m2. The inlet temperatures of the test module were set in the range of 5-85 °C. Visualization results showed that the flow patterns can be classified into bubbly flow, combined bubble flow and stratified slug flow. The flow mode tended to transition to stratified flow as the inlet temperature increased, and the combined bubble flow was found to be absent at high inlet subcooling. The size of the nucleated bubbles increased with the increase of inlet temperature during bubbly flow. The length to width ratio, survival time of nucleated bubbles and the bubble dynamic behaviors were respectively discussed. The heat transfer characteristics and pressure drop were strongly dominated by the two-phase flow patterns. The optimum flow boiling heat transfer was achieved when the flow pattern transitioned to combined bubble flow and early stage of stratified slug flow, during which bubble nucleation and thin liquid film evaporation jointly contributed to the heat dissipation. The pressure drop increased with the increase of inlet subcooling because of the greater liquid viscosity, and the sudden spurt of pressure drop was observed due to the occurrence of stratified slug flow. This work provides a preliminary investigation of two-phase flow boiling performance in large aspect ratio open microchannels, which may provide important insights of bubble dynamics or interfacial mechanics in open microchannel heat sinks.

Experimental investigation on flow boiling characteristics of HFO-1234yf in a 0.5 mm microchannel

The flow boiling heat transfer of environmentally friendly refrigerant HFO-1234yf in a horizontal circular microchannel is experimentally investigated. The inner diameter and heating length of the test channel are 0.5 mm and 300 mm, respectively. Experiments are conducted at relatively high mass flux ranging from 750 to 1500 kg m−2s−1, heat flux from 25 to 85 kW m−2 and inlet pressure between 0.604 and 0.634 MPa. The heat flux is stepwise imposed to the test channel until dryout occurs. The flow boiling heat transfer characteristics are analyzed in detail. In the pre-dryout region, the heat transfer coefficient (HTC) is found to be mainly determined by the heat flux, while the effect of mass flux is weak. Two different HTC behaviors are observed at low and high mass fluxes, which can be attributed to the prominent effect of thin liquid film evaporation on heat transfer and the occurrence of partial dryout. Moreover, the oscillation of mass fluxes and wall temperatures are observed at high mass flux conditions, resulting in heat transfer deterioration. In the post-dryout region, the critical heat flux (CHF) is found to increase with mass flux, while the critical vapor quality would decrease. Finally, the experimental data of HTC and CHF are compared with correlations from the literature and good agreements are presented.

Three-dimensional numerical study on evaporating two-phase flow in heated micro T-junction

A three-dimensional numerical model was developed to study the flow structure, bubble dynamics, and heat transfer of an evaporating two-phase flow in a heated micro T-junction. The numerical model was validated against both experimental and simulation data available in the literature. The influences of the inlet Reynolds number (Re) and seed bubble volume (V0) were quantitatively evaluated. The flow structure gradually changed from a smooth flow to a twining flow as the Re increased. The vortices disappeared and the flow became smoother during the transit of the bubble. Two competing mechanisms influencing the local heat transfer were recognised: the suppression of convection heat transfer caused by bubble obstruction and the enhancement in heat transfer caused by thin liquid film evaporation. The heat transfer enhancement decreased monotonically as the Re increased or the V0 decreased, and the main channel was more sensitive to variations in the V0.

Experimental analysis of the evaporation of a thin liquid film deposited on a capillary heated tube: estimation of the local heat transfer coefficient

The aim of this work is to estimate the local heat flux and heat transfer coefficient for the case of evaporation of thin liquid film deposited on capillary heated channel: it plays a fundamental role in the two-phase heat transfer processes inside mini-channels. In the present analysis it is investigated a semi-infinite slug flow (one liquid slug followed by one single vapour bubble) in a heated capillary copper tube. The estimation procedure here adopted is based on the solution of the inverse heat conduction problem within the wall domain adopting, as input data, the temperature field on the external tube wall acquired by means of infrared thermography.

A novel model of liquid film flow and evaporation for thermal protection to a chamber with high temperature and high shear force

Thin liquid film evaporation is a prevailing cooling technique for thermal protection to the inner wall of a chamber with high-intensity combustion. A novel model is developed to investigate the flow and evaporation of liquid film in a rocket combustion chamber with high temperature and high shear force, in which shear stress at the interface between liquid film and high temperature combustion gas with high velocity, capillary pressure of liquid and disjoining pressure associated with liquid film evaporation are taken into account. A numerical algorithm of the backward solution is deliberately adopted to solve this model so as to eliminate the entrance effect caused by different inlet conditions of liquid film. For given temperature and hot gas rate in a chamber, a stably evolution regime of liquid film with the thickness from thick to thin is depicted corresponding to the total process of being sheared into film and being evaporated to dryness. Results indicate that the three forces mentioned above play critical parts in flowing and evaporating of liquid film during its different evolution regimes. It was shown that shear stress and capillary pressure have a positive effect on the flow and evaporation of film, as well as a negative effect from the disjoining pressure. The effects of these forces are different dependent on different initial average thickness of film, and the distribution profile of these forces are totally illustrated in detailed. The prediction of liquid film length, from being injected to being evaporated, by the proposed model is in a good agreement with available experimental data. All findings are expected to provide a fundamental guideline for the design and optimization of efficient thermal protect to the combustion chamber of space engine.

Effect of Marangoni Convection on the Perovskite Thin Liquid Film Deposition.

Motivated by recent advances in the development of thin-film perovskite solar cells, the evaporation and deposition of a perovskite thin liquid film on a hydrophilic substrate, also in some cases subjected to ultrasonic vibrations, are studied in this paper. In practice, in the literature, the complexity of the underlying phenomenon has led to the study of a thick (macroscale) liquid film in the absence of solidification. Here, we investigate evaporation mechanics of a thin (microscale) liquid film of perovskite solution. We demonstrate flow fields within the film and study the reason behind the formation of such flow patterns within a thin liquid film and attribute such flows to surface-tension-induced Marangoni flows and rule out the possibility of the presence of buoyancy-induced Rayleigh-Benard convection. We show that perovskite deposition starts at the film contact lines and propagates toward the film center, creating two regions of the perovskite film with distinct characteristics. Our thermography results (temperature mapping) of the film surface show agreement between the temperature map and the flow patterns on the film surface. Moreover, we show that imposing ultrasonic vibration on an evaporating liquid film results in a more uniformly distributed flow pattern across the film due to micromixing enhancement achieved by ultrasonic vibrations.

Enhanced flow boiling in microchannels by incorporating multiple micro-nozzles and micro-pinfin fences

Phase-change heat transfer is a promising approach for high-power electronics cooling. However, the chaotic two-phase transport and dominant laminar flow in microchannels inhibits flow boiling performance. Thus, the ability to coordinate the two-phase transport in a highly favorable fashion would be highly desired. In this study, a new microchannel configuration by incorporating capillary micro-pinfin fences and multiple micro-nozzles has been proposed to sustain thin liquid film evaporation, promote mixing and global liquid supply simultaneously. A new boundary layer covered with thin liquid film is activated by the capillary micro-pinfin fences along the sidewalls of the channel. Additionally, the sustainable thin liquid film is maintained using capillary effect, which can promote the capillary-driven flow inside the gap between micro-pinfin fences and the sidewalls. As a result, significantly enhanced flow boiling has been achieved on both DI-water and HFE-7100. A critical heat flux (CHF) up to 944 W/cm2 has been demonstrated using DI-water at a mass velocity of 600 kg/m2 s, accounting for a 43% enhancement compared to the configuration of multiple micro-nozzles without capillary micro-pinfin fences. Compared to the base configuration of plain wall microchannels, the enhancement of CHF is over threefold at a mass velocity of 389 kg/m2s. Equally important, this new microchannel configuration works well on dielectric fluids, which is challenging to promote CHF due to their unfavorable thermophysical properties. A CHF of 287 W/cm2 with a noticeable enhancement of 56% has been achieved at a mass velocity of 2772 kg/m2 s on HFE-7100 at room temperature. Moreover, the enhancements of heat transfer coefficient (HTC) on HFE-7100 are more noticeable compared to that of DI-water. For example, 120% higher HTC at a moderate mass velocity of 693 kg/m2 s has been realized. All the enhancements are achieved without the escalating pressure drop.

Direct local heat flux measurement during water flow boiling in a rectangular minichannel using a MEMS heat flux sensor

Heat transfer characteristics of water flow boiling in a minichannel were investigated through the direct measurement of local heat flux using a fabricated MEMS heat flux sensor with a multi-layered structure. Local temperatures and heat fluxes in the water flow boiling were measured at a sampling frequency of 10 kHz in synchronism with the high-speed visualization of the boiling behavior. The measurement results revealed fundamental heat transfer processes, including thin liquid film evaporation, dry-out of the liquid film, transient heat conduction after dry patch rewetting, and single-phase forced convection. The thin liquid film evaporation indicated a high local heat flux that was well over 1 MW/m2 and provided a dominant contribution to the wall heat transfer. Furthermore, the post-rewetting transient heat conduction indicated a high heat flux that was higher than 1 MW/m2. However, it was insignificant in terms of the overall wall heat transfer at all the tested heat fluxes because of its short duration. The contribution of liquid-phase forced convection was also small, except at low vapor qualities.

A dynamically controllable evaporative cooling system for thermal management of transient heat loads

Evaporation of thin liquid films has the potential to sustain significantly high heat fluxes. This cooling mechanism has been studied extensively to create devices capable of handling large static heat loads for electronic cooling. However, significant and sudden variations in the heat loads can result in the dryout of the thin film, which can jeopardize the performance of the cooling system causing irreversible damage to the electronic component. Hence, a mechanism capable of controlling the performance of evaporative cooling is necessary. This study presents a unique strategy that combines a novel evaporator with a vapor compression cycle, whose performance can be varied to address the transient heat loads. The evaporator is designed to take advantage of simultaneously-occurring multiple modes of heat transfer to dissipate large heat fluxes. This study presents a device and system-level analysis to quantify the effect of different system operating conditions that control the overall cooling performance and efficiency.

Flow boiling in horizontal multiport tube: Development of new heat transfer model for rectangular minichannels

The flow boiling characteristics of R32 and R1234ze(E) were investigated in a horizontal multiport tube with rectangular minichannels. The local heat transfer coefficient and frictional pressure drop were measured under mass fluxes of 50–400 kg m−2 s−1 at a saturation temperature of 15 °C. The heat transfer was characterized by forced convection in the region with a low heat flux, high mass flux, and high vapor quality; the heat transfer was characterized by nucleate boiling in the region with a high heat flux and a low vapor quality. Moreover, the heat transfer was characterized by the thin liquid film formed around the elongated vapor plugs with a low mass flux and low heat flux. These heat transfer mechanisms were also confirmed based on the visualization results of the flow boiling. The heat transfer deteriorated in the plug and slug-annular flows due to the occurrence of dry patches. A new heat transfer model for multiport rectangular minichannels was developed that considered forced convection, nucleate boiling, and thin liquid film evaporation. Moreover, this new heat transfer model considered flow patterns inside the multiport minichannels, heat transfer deterioration due to dry patches, and dryout inception quality. The newly model can predict the heat transfer characteristics in multiport rectangular minichannels for a mean deviation of less than approximately 15%. The frictional pressure drop of R32 was smaller than that of R1234ze(E) under the same mass flux and vapor quality conditions and compared with previous correlations.

Evaporation of thin liquid film of single and multi-component hydrocarbon fuel from a hot plate

Evaporation of liquid film is of great importance, especially to the automotive industry, owing to its prevalence in automotive wet components. Thin liquid films usually deposit/spread in the inlet ports of engines using port fuel injection (PFI), at the nozzle tip of gasoline direct injection (GDI) engines, on the piston head of GDI and diesel engines, etc. It is important to predict the evaporation rate of these liquid films which have the thickness of the order of micrometers and may potentially lead to carbon deposits and consequent emissions. In this work, results of extensive experimental and numerical investigations of the evaporation of single and multi-component hydrocarbon film are presented. Liquid films are created by spraying the fuel on a smooth aluminum plate, and its evaporation pattern is studied at various temperatures of the plate. The film thickness reduction was monitored over time using a confocal point probe until the liquid film is completely evaporated. A one-dimensional numerical model is developed to predict the reduction of film thickness due to evaporation for single and multi-component fuels. The numerical results compared well with the measurements.

Boiling heat transfer, pressure drop, and flow pattern in a horizontal square minichannel

This study experimentally investigated the flow boiling heat transfer, pressure drop, and flow pattern in a horizontal square minichannel with a hydraulic diameter of 2.0 mm, and the effects of mass flux, vapor quality, heat flux, and refrigerant properties on the flow boiling characteristics were clarified. The heat transfer coefficient and pressure drop of R32 and R1234yf were measured in a mass flux range of 50–400 kgm−2s−1 at a saturation temperature of 15 °C. The flow pattern of the square minichannel outlet was observed and was classified as plug, wavy, churn, and annular flows. The heat transfer coefficients in the square minichannel were larger than those in the circular minichannel with a similar hydraulic diameter at low mass flux conditions. The heat transfer coefficients of R32 indicated higher values compared with those of R1234yf at same mass flux and qualities. An empirical heat transfer model taking into account the forced convection, nucleate boiling, and thin liquid film evaporation was developed for horizontal square and circular minichannels. The frictional pressure drop of R32 was 1.5–2 times higher than that of R1234yf at same mass flux and vapor quality condition, and the effect of channel shape on the frictional pressure drop was small unlike the boiling heat transfer.

Thermographic characterization of thin liquid film formation and evaporation in microchannels.

The science of transport in microchannels has greatly benefited applications ranging from micro-mixing, chemical synthesis and biological analysis to compact and efficient energy devices. One of the most critical and intricate phenomena in this field of science is the dynamics of thin liquid film formation during the flow of liquid and gas/vapor mixtures. These films can form in microseconds and be less than a micrometer thick, while dominating thermal transport in phase-change processes. Here, we report the captured details of these phenomena using a new measurement technique with unprecedented spatial and temporal resolutions of 20 μm and 100 μs, respectively. Thin films with thicknesses ranging from 1 to 20 μm forming around elongated bubbles over a capillary number range of 0.025 to 0.1 are characterized. The measurements suggest that these films thermally develop and evaporate at timescales in the order of 1-10 ms, two orders of magnitude longer than their formation timescale. The formation, reflow and evaporation of the liquid film constitute a complex dynamic involving variations of the film thickness over the periphery of a rectangular channel, leading to a thicker liquid film feeding (through lateral capillary wicking) a much thinner rapidly evaporating film. As a result, the thinner film dictates the rate of surface heat transfer while the thicker film determines the duration of thin film evaporation. A modified Bretherton model provides the best fit to the experimental results.

Characteristics of phase-change heat transfer in a capillary evaporator based on microscale infrared/visible observation

In this study, the characteristic of liquid–vapor interface behavior in a capillary evaporator of loop heat pipes (LHPs) is studied experimentally and theoretically. The behavior of a phase–change heat transfer at the surface of a fine porous medium that is called a wick, is evaluated using microscopic infrared camera and microscope. Four distinct samples made using four different materials (polytetrafluoroethylene and stainless steel (SS1.0 μm, SS4.5 μm, SS22 μm)) were used to simulate a part of the wick used in the evaporator. The changes in a heat transfer coefficient were caused by three types of liquid–vapor interface behaviors. In the analysis, a one-dimensional model of the liquid–vapor interface behavior in the wick is proposed based on thin liquid film evaporation theory. The predicted heat transfer coefficient was in good agreement with the experimental data.

Heat Transfer Characteristics during Boiling of Immiscible Liquids Flowing in Narrow Rectangular Heated Channels

The use of immiscible liquids for cooling of surfaces with high heat generation density is proposed based on the experimental verification of its superior cooling characteristics in fundamental systems of pool boiling and flow boiling in a tube. For the purpose of practical applications, however, heat transfer characteristics due to flow boiling in narrow rectangular channels with different small gap sizes need to be investigated. The immiscible liquids employed here are FC72 and water, and the gap size is varied as 2, 1 and 0.5mm between parallel rectangular plates of 30mm×175mm, where one plate is heated. To evaluate the effect of gap size, the heat transfer characteristics are compared at the same inlet velocity. The generation of large flattened bubbles in a narrow gap results in two opposite trends of the heat transfer enhancement due to thin liquid film evaporation and of the deterioration due to the extension of dry patch in the liquid film. The situation is the same as that observed for pure liquids. The latter negative effect is emphasized for extremely small gap sizes if the flow rate ratio of more-volatile liquid to the total is not reduced. The addition of small flow rate of less-volatile liquid can increase the critical heat flux of pure more-volatile liquid, while the surface temperature increases at the same time and assume the values between those for more-volatile and less-volatile liquids. By the selection of small flow rate ratio of more-volatile liquid, the surface temperature of pure less-volatile liquid can be decreased without reducing high critical heat flux inherent in the less-volatile liquid employed. The trend of heat transfer characteristics for flow boiling of immiscible mixtures in narrow channels is more sensitive to the composition compared to the flow boiling in a round tube.