Vapor Mass Flux Research Articles

The influence of a vertical chevron corrugated plate on wetting and thermal performance of a detachable plate-type falling film absorber

The absorption system utilizing LiBr/H2O as working fluids plays an increasingly important role in district heating. However, the heat transfer of the existing systems is mainly through the shell-and-tube structure, resulting in the system’s vast land occupation. A detachable plate-type falling film absorber is proposed to address the problem. The type and geometry of the plate utilized in this system are designed in this paper. The vertical chevron corrugated plate is proposed to improve the wetting performance in the negative pressure channel and enhance the heat transfer performance of the water in the positive pressure channel. The thermal performance experiment of the system with this plate is carried out under the absorption-evaporation condition. Under the rated condition, the overall heat transfer coefficient of absorption process rises to 0.983 kW·m−2·K−1, and the mass flux of vapor absorbed rises to 1.9 × 10−3 kg·m−2·s−1. Moreover, the overall heat transfer coefficient of the plate falling film absorption process is split, and the empirical correlations of convective heat transfer on the water side and the solution side are fitted respectively. The thermal performance improvement further improves the feasibility of miniaturization of the absorption system.

Condensation heat transfer and pressure drop characteristics of Isobutane in horizontal channels with twisted tape inserts

• Using twisted tapes augmented the heat transfer and pressure drops. • The heat transfer rate enhanced by giving rise to vapor quality and mass flux. • The pressure losses increased by giving rise to vapor quality and mass flux. • The performance factor varied between 0.39 to 1.05 using inserts. The enhancement of heat transfer rate in heat exchangers has been considered a major concern. In this research, the impacts of inserting twisted tapes in horizontal two-phase flow heat exchangers are discussed, and different values of vapor qualities (in the range of 0.1 to 0.7) and mass velocities (in the range of 119 to 367 kgm −2 s −1 ) are considered during forced convective condensation of R600a (Isobutane). The test case is a pipe made from copper with an inner diameter of 8.1 mm and a length of 1000 mm. Furthermore, three twisted tape inserts with various twist ratios (defined as the ratio of the twisted tape pitch to the test pipe inner diameter) of 4, 10, and 15 are used. The results illustrated that installing twisted tapes results in the increment of pressure drops and the rate of heat transfer in comparison to the smooth case. Furthermore, the pressure drops and heat transfer rates augment as the refrigerant mass velocity and vapor quality increase. Depending on the inserts type and operating conditions, the performance factor (a criterion to assess the performance modification compared to the primary test case) between 0.39 to 1.05 was obtained. It was also observed that there exists an optimum amount of the refrigerant mass velocity at which the performance factor is higher. Results showed that generally using twisted tapes in heat exchangers is not instrumental unless when the main concern is to improve the heat transfer rate or when the augmented power consumed by pump can be justified.

Effect of gas mixture on temperature and mass streaming in a phase-change thermoacoustic engine

Wet thermoacoustic engines (WTEs) are simple, robust energy conversion devices that employ a condensable vapor to generate acoustic power from heat. While previous studies focused on the potential of WTE technology to convert low-grade heat to power, this work focuses on the WTE's dependence on the working gas mixture, unique mass transport, and low working temperature properties, which make it a potential candidate for thermally driven separation processes, e.g., distillation and water desalination. In a set of experiments, the steady-state behavior of a water-based, standing-wave WTE was studied. The results highlight a remarkable trait of this system—a WTE is capable of producing a substantial flux of water vapor while exhibiting an operating hot-side temperature significantly lower than the boiling point. This “limiting temperature” is independent of the heat input to the system—theoretically predicted in systems that have no external “load” that consumes the acoustic power. In addition, the results suggest that the characteristics of the working gas play an important role in determining the device behavior. It was shown that in the tested range, a heavier, more complex-structured gas has significantly improved the steady-state performance of the engine. Specifically, a WTE working with SF6 as its working gas exhibited a vapor mass flux nearly three times greater than simple evaporation at the same heating power while maintaining a temperature as low as 57 °C. The link between gas characteristics and engine behavior is outlined. In conclusion, potential applications of the device and future research avenues are discussed.

Assessing the impact of disjoining pressure on thin-film evaporation with atomistic simulation and kinetic theory

Molecular dynamics (MD) simulations are applied to validate the Hertz–Knudsen–Schrage (HKS) relation for the evaporation mass flux in the presence of disjoining pressure. A non-equilibrium MD simulation system is designed to directly extract the evaporation mass flux for a Lennard-Jones fluid. The temperatures, pressures, properties, and disjoining pressure required to evaluate the HKS relation are obtained from the non-equilibrium MD simulation and complementary equilibrium MD simulations. The direct MD evaporation mass flux and that from the HKS relation agree within 10%. We define an evaporation Kapitza length that estimates where the liquid conduction and evaporation thermal resistances are equal. We propose that surface structures that promote film thicknesses comparable to or smaller than the evaporation Kapitza length, but larger than the adsorbed film thickness, will maximize the evaporation rate.

Condensation frictional pressure drop characteristic of R-600a inside the horizontal smooth and dimpled helical coiled tube in shell type heat exchanger

In this paper, an experimental study on condensation frictional pressure drop of R-600a inside smooth and dimpled helical coiled tubes were examined. The experiments were performed at saturation temperature of 35 °C, and 45 °C with mass fluxes of 75, 115, 156 and 191 kg m−2s −1. The effect of vapor quality, mass flux and saturation temperature on the frictional pressure drop were inspected. Moreover, the results obtained from smooth and dimpled helical coiled tube were also compared from those smooth straight tube at same operating condition. The results showed that the dimpled helical coiled tube offers higher frictional pressure drop compared to the smooth straight tube and smooth helically coiled tube. Finally, the correlations have been proposed to predict the frictional pressure drop during condensation of R-600a inside smooth and dimpled helically coil tube that predict the data within ±20%.

Diabatic visualization study of R245fa two phase flow pattern characteristics in horizontal smooth and microfin tube

This study investigates the flow pattern characteristics of R245fa in the smooth and microfin tubes under diabatic conditions. A novel electromagnetic induction system was designed to obtain visualization data in the transparent test section with a controlled heat flux. The experiments were conducted with mass fluxes of 50–240 kg/m2 s, heat fluxes of 5.65–14.87 kW/m2 and a saturation pressure Psat = 0.2 MPa. Six flow patterns were identified in both smooth and microfin tubes. The effects of the vapor quality, mass flux and heat flux on the flow patterns were analyzed. Compared with the smooth tube, the transition from intermittent to wavy-annular flow occurred at lower vapor quality in the microfin tube. Moreover, the liquid film was spread, and the shrinkage of the heated thin film was restrained in the microfin tube, so it can significantly improve the evaporation heat transfer performance. Six predictive models were compared with the present flow pattern map. The Frl = cXttn format criterion was suitable for the intermittent/wavy-annular transition of R245fa in horizontal tubes.

Experimental Study of Flow Boiling Heat Transfer of R30 in Microchannels

Two-phase cooling technique including flow boiling heat transfer in microchannel is being widely applied in the mechanical and electronic devices, whereas most of the existing related studies still experiment with common coolants with low boiling points that are not suitable for extreme high-temperature heat dissipation. Thus the flow boiling heat transfer characteristics of R30 have been analyzed through an experiment in parallel microchannels with diameters of 0.5 and 1.0 mm. The experiment was performed under the following conditions: heat flux, ; mass flux, ; outlet vapor quality, 0–0.52. Results showed that the heat transfer coefficient is determined by heat flux and vapor quality in low-vapor-quality region dominated by nucleate boiling, and affected by vapor quality and mass flux in high-vapor-quality region dominated by convective boiling. Intermittent dryout occurs in the nucleate boiling region and is affected by mass flux resulting from the suppression of nucleate boiling. Higher mass flux can relieve the intermittent dryout and postpone the onset of the dryout incipience to higher vapor quality. Comparison with previous correlations showed their unsatisfactory accuracy on present data and implied relatively well heat transfer performance of R30.

Pressure drop and heat transfer of flow boiling R134a in a dimpled flat duct

A test facility has been built up to investigate the pressure drop and heat transfer of flow boiling R134a in a dimpled flat duct. The experiments are conducted at mass flux from 102.2 to 205.0 kg m−2 s−1, heat flux from 2.3 to 9.3 kW m−2, pressure from 349.8 to 488.4 kPa and vapor quality from 0.05 to 0.95. The results show that the frictional pressure gradient increases with increasing vapor quality and mass flux, while decreases with a rise in pressure. The heat flux has little impact on the frictional pressure gradient. The flow boiling heat transfer coefficient decreases slightly first and then increases significantly with an increase in vapor quality, which is attributed to the combined contributions of nucleate boiling heat transfer and convective evaporation heat transfer. No dryout has been observed in the tested vapor quality region. The flow boiling heat transfer coefficient visibly increases with increasing mass flux and heat flux, while the rise in pressure just slightly increases the flow boiling heat transfer coefficient in low vapor quality region only. The comparisons with correlations for regular channels show that the dimpled flat duct has 3.9 to 8.3 times frictional pressure gradient and 1.3 to 3.0 times heat transfer coefficient.

Synergistic Tandem Solar Electricity-Water Generators

Summary Energy and clean water are two intertwined fundamental elements for human civilization and sustainable development. Here, we devise a monolithic tandem solar electricity-water generator that synergistically produces electricity and clean water by utilizing the full spectrum of solar irradiance. The tandem generator has two components: a top infrared-transparent photovoltaic device using above-band-gap photons, and a bottom solar water purifier using the below-band-gap photons. More strikingly, a well-designed water-proof thermal interconnecting layer (WTIL) makes these two components work synergistically: the bottom purifier serves as an evaporative cooler of the top solar cell to increase its efficiency, whereas the thermalization energy of the top cell is reutilized by the bottom purifier for producing more clean water. We experimentally demonstrate that a prototype hybrid tandem solar device with WTIL can generate electricity with a power output of 204 W m−2 and purify water at a rate of 0.80 kg m−2 h−1 under 1-sun illumination.

Temperature Discontinuity at an Evaporating Water Interface

Evaporative mass flux is governed by the interfacial state of liquid and vapor phases. For closely similar pressures and mass fluxes of liquid water into its own vapor, the discontinuity between in...

Experimental analysis of condensation heat transfer and frictional pressure drop in a horizontal circular mini channel

The present study aims to investigate the effect of working fluid, mass flux, vapour quality and saturation temperature on local condensation Heat Transfer Coefficient (HTC) and local Frictional Pressure Drop (FPD) in a horizontal circular mini-channel. The refrigerants HFO-1234yf and HFC-R134a have been used as a working fluids. The mass flux was varied from 200 to 800 kg/m2 s whereas two distinct saturation temperatures were used: 35 °C and 40 °C. The experimental analysis was carried out using horizontal circular single port mini-channel of hydraulic diameter 1 mm. During experiment, inlet condition of refrigerant was maintained to be saturated vapour. The results show that HTC and FPD increases with increase in vapour quality and mass flux whereas decreases with increase in saturation temperature. The experiment results of present study and previously published results are compared with various predictive models. Models which show minimum deviation from experimental results were used to develop new modified model to predict HTC with MARD of ± 15 % and FPD with MARD of ± 10 %.

CNT–PVDF freestanding sheets for direct solar evaporation toward continuous desalination applications

Typically, solar desalination membranes are fabricated through non-scalable techniques and consist of multiple layers (a photo-absorber layer deposited on a hydrophilic substrate), resulting in multistage evaporation processes, influencing the efficiency of the process. Moreover, the classical approach is to test such developed membranes in a batch-mode setup, in which the system is stagnant; motivating the need to investigate their applicability in continuous flow setups to expand on the commercial outreach criteria. Herein, we investigate freestanding, thin, and nanoporous composite membranes for direct solar-assisted evaporation experiments in an in-house-built continuous flow thin-film evaporation device. Our membrane fabrication methodology had three notable advantages: (1) the utilization of blackbody materials, (2) the achievement of variable membrane properties, and (3) the utilization of a single composite freestanding membrane. We show that evaporation mass flux (assisted with 1 sun of direct solar irradiation) increased as mean pore size and membrane thickness decreased, while volume porosity, solar absorption, and wettability increased. Out of all of the membranes investigated, the membrane modified with the least amount of PVDF exhibited the highest evaporation flux (0.55 L m−2 h−1) and the highest solar efficiency (~ 38.4%).

Two-phase flow pattern and heat transfer coefficients of R245fa/R134a under non-uniform heat flux

The non-uniform distribution of the heat flux commonly occurs in systems where vapor-liquid phase changes, such as nuclear reactors, and aerospace thermal control systems. This poses a great challenge on the study of pressure drop, heat transfer coefficient, and flow pattern prediction. In addition, flow patterns and heat transfer characteristics are usually affected by differences in fluid properties (viscosity, density, and surface tension), thermodynamic conditions (pressure, temperature), and geometric dimensions. At present, most of the researches on non-uniform heat flux are concentrated in systems with pure working fluids. However, compared to the pure working fluid, the mixed working fluid, particularly the zeotropic working fluid can improve the cycle efficiency and has great application potential, chiefly due to the temperature glide. There are few studies on the two-phase flow pattern and heat transfer characteristics of the zeotropic working fluid under non-uniform heat flux. Aiming at the problem of vapor-liquid phase change of zeotropic working fluid under non-uniform heat flux boundary conditions, a two-phase flow boiling experimental system in a horizontal tube with a 10 mm inner diameter under non-uniform heat flow was established. The studied zeotropic working fluid is R245fa/R134a with a mass fraction of 0.7:0.3. Experiments were performed at conditions where the mass flux ranges from 175 to 373 kg/(m2 s), while the heat flux ranges from 9.95 to 47.57 kW/m2. Using a high-speed camera and image processing technology, the gray values of the observed bubble flow, plug flow, stratified flow, and annular flow were analyzed, and the flow patterns under both non-uniform and uniform heat flux were demonstrated. Effects of the vapor quality, mass flux, heat flux and saturation temperature on the two-phase heat transfer coefficient were analyzed. And on the basis of experimental data, the prediction accuracy of four commonly used correlations was evaluated. The results show that the gray value of bubble flow and plug flow fluctuate widely, while the gray value fluctuation range of stratified flow and annular flow is relatively small. The typical flow pattern under non-uniform heat flux is the same as flow pattern under uniform heat flux. With the same amount of the additional heat, under the condition of non-uniform heat flux, the initial vapor quality of annular flow is advanced, and the proportion of the annular flow is larger than the non-annular flow. The main reason for this phenomenon is that the flow instability caused by non-uniform heat flux accelerates the transition of intermittent flow to annular flow. In addition, the heat transfer coefficient changes little at the lower vapor quality, and the heat transfer coefficient of the intermittent flow region is independent of vapor quality. In the annular flow region with higher vapor quality, the heat transfer coefficient increases with the increase of vapor quality. The heat transfer coefficient increases with the increase of mass flux and heat flux. The saturation temperature has little effect on the heat transfer coefficient in the convection boiling region and has a certain influence in the nucleate boiling region. Furthermore, the prediction ability of the four correlations is poor in the non-annular flow region. Relatively, the prediction accuracy of the Sun & Mishima correlation is the highest in the annular flow region.

Numerical investigation of nanoporous evaporation using direct simulation Monte Carlo

Evaporation is an effective cooling mechanism widely exploited in the thermal management of modern electronic devices, with a growing interest in the evaporation process in thin film based nanoporous membrane technologies. At such scales, classical approaches fail and one requires solutions of the Boltzmann equation; these are obtained here using the direct simulation Monte Carlo method. In particular, the evaporation from representative nanoporous meniscus shapes, corresponding to different operating conditions, has been investigated. Evaporation rates for the different conditions have been characterized as a function of a wide range of Knudsen numbers and free-stream Mach numbers. Additionally, the influence of porosity and evaporation coefficient on the nanoporous evaporation rates has been assessed. Investigations have also been carried out to consider cases where the meniscus has sunk within the pore, and cooling efficacy compared with cases where the meniscus is pinned to the top of the pore. This work demonstrates that the net evaporative mass flux is ultimately determined by the interplay between various physical effects, whose dominance is quantified by the Knudsen number, porosity, evaporation coefficient and the meniscus shape. This work thus provides useful information for the design of nanoporous membrane-based cooling devices.

Experiment and simulation on downward flow condensation of nitrogen in vertical tubes

The heat transfer characteristics of nitrogen downward flow condensation are measured in 2 mm and 1 mm vertical tubes. The saturation temperature of nitrogen ranges from 100.4 to 108.0 K. The mass flux varies from 32.7 to 78.6 kg/m2 s for 2 mm tube and from 52.4 to 314.4 kg/m2 s for 1 mm tube. Experimental results of the flow condensation heat transfer coefficient are proposed and analyzed for different vapor qualities, mass fluxes, tube diameters and saturation temperatures. An annular condensation model with the consideration of the interfacial shear stress and the gravity is proposed based on the assumption of uniform film thickness around the tube’s circumference, and the model is validated by the experimental data. Based on the model, the flow and heat transfer characteristics, including the thickness of the liquid film and the condensation heat transfer coefficient are analyzed. Finally, the model and several previous correlations of flow condensation heat transfer coefficient are evaluated by the comparison with the experimental data.

Atomic insights of thin‐film evaporation over biphilic surfaces: Effect of philic‐phobic patterning and wettability contrast

AbstractMolecular dynamics simulations have been carried out to investigate the effect of philic‐phobic patterning and wettability contrast of biphilic (surface with interlaced hydrophilic and hydrophobic segments) surfaces on thin‐film evaporation and compare its performance with the surfaces with uniform wettability, ie, hydrophilic/hydrophobic. Thin liquid film of argon with 3 nm film thickness is placed over platinum surface. After equilibrating the system at 90 K, the temperature of the platinum wall is raised to two different temperatures 110 and 130 K to study the effect of wall superheat temperature. The results obtained in this study indicate that biphilic surface offers wall heat flux and evaporative mass flux close to the hydrophilic surface. With the decrease of philic‐phobic pattern bandwidth of biphilic and superbiphilic surface, the maximum value of evaporative mass flux and wall heat flux increases. Also higher wettability contrast among hydrophilic and hydrophobic region offers higher value of wall heat flux and evaporative mass flux. The boiling inception time decreases with the decrease of philic‐phobic pattern bandwidth and increase of wettability contrast therefore, the nonevaporating layer appears earlier in case of lower philic‐phobic pattern bandwidth and higher wettability contrast.

Two-phase flow patterns and pressure drop of a low GWP refrigerant R-1234ze(E) in a plate heat exchanger under adiabatic conditions

The flow characteristics and flow pattern maps of R-1234ze(E) in a plate heat exchanger (PHE) are investigated at various vapor qualities and mass fluxes through flow visualizations. In the two-phase flow of R-1234ze(E) in the PHE, five dominant flow regimes are observed according to the vapor quality: slug flow, wavy liquid film flow, pulsating annular flow, vapor-preferred path annular flow, and entire annular flow. The flow characteristics of each flow regime in the PHE are analyzed in detail. The flow patterns in the PHE differ considerably from those in the smooth circular tube (SCT) owing to the corrugation and channel geometry of the PHE. The asymmetric flow observed in the PHE is explained with liquid and vapor-preferred paths. The flow pattern maps in the PHE and SCT are developed and compared based on the visualization results. Additionally, the relationship between the frictional pressure drop and the flow regimes is determined to be trivial, whereas the fluctuation in the pressure drop has a strong dependence on the flow regimes owing to intermittent flooding.

Molecular dynamics study about the effects of random surface roughness on nanoscale boiling process

The boiling process of argon above solid copper substrates with different surface roughness levels are investigated using molecular dynamics simulations. The randomly distributed rough surface profiles are generated by a multivariable Weierstrass-Mandelbrot function. With the application of the thermostat on the solid atoms, bubble nucleated at the solid-liquid interface after experiencing a period of evaporation process. Afterwards, the bubble grew into vapor film and pushed the bulk liquid cluster away from the solid wall. It was found that the rough structures advanced the boiling inception time and increased the evaporation mass flux. The existence of cavities is favorable for bubble nuclei appearance. Besides, the heat transfer between the solid and fluid atoms become more efficient with the help of surface nanostructures. The energy exchange rate is further improved with higher surface roughness.

Evolution of supernovae-driven superbubbles with conduction and cooling

ABSTRACT We use spherically symmetric hydrodynamic simulations to study the dynamical evolution and internal structure of superbubbles (SBs) driven by clustered supernovae (SNe), focusing on the effects of thermal conduction and cooling in the interface between the hot bubble interior and cooled shell. Our simulations employ an effective diffusivity to account for turbulent mixing from non-linear instabilities that are not captured in 1D. The conductive heat flux into the shell is balanced by a combination of cooling in the interface and evaporation of shell gas into the bubble interior. This evaporation increases the density, and decreases the temperature, of the SB interior by more than an order of magnitude relative to simulations without conduction. However, most of the energy conducted into the interface is immediately lost to cooling, reducing the evaporative mass flux required to balance conduction. As a result, the evaporation rate is typically a factor of ∼3–30 lower than predicted by the classical similarity solution of (Weaver et al. 1977), which neglects cooling. Blast waves from the first ∼30 SNe remain supersonic in the SB interior because reduced evaporation from the interface lowers the mass they sweep up in the hot interior. Updating the Weaver solution to include cooling, we construct a new analytic model to predict the cooling rate, evaporation rate, and temporal evolution of SBs. The cooling rate, and hence the hot gas mass, momentum, and energy delivered by SBs, is set by the ambient interstellar mass density and the efficiency of non-linear mixing at the bubble–shell interface.

Kinetics of thermal hydrolysis of crude palm oil with mass and heat transfer in a closed system

Crude palm oil (CPO) is the main product of oil palm processing and is obtained through a series of unit operations. In a palm oil mill, the main unit operations involve applying saturated steam in the steriliser and digester. The high temperature and water content used will induce thermal hydrolysis, leading to accumulation of free fatty acids (FFAs), the levels of which are considered a major quality index of CPO. A detailed study of the thermal hydrolysis of CPO at different temperatures and water contents in a closed-vessel system was carried out. The results show that there was an increase in FFA accumulation as temperature and initial water content increased. A kinetic model of the hydrolysis was constructed taking into account heat and mass transfer phenomena. The important parameters of the model were the reaction frequency factor (k0T) and the power factor of water fraction (n), which were successfully estimated by gPROMS ModelBuilder at 2.55×10–6 (m3/kmol)0.85min−1 and 0.62, respectively. A sensitivity analysis revealed that the simulated profiles (i.e. FFA content and vapour mass flux, W˙WV) were highly sensitive to these parameters. The model can be a useful tool in the further redesign and quality improvement of the industrial of CPO extraction processes.