Vapor Mass Flux Research Articles

Advancing thermohydraulic performance of micro-grooved flat heat pipes through a combined numerical-experimental approach

A combined numerical-experimental model has been developed to predict the thermohydraulic characteristics of micro-grooved flat heat pipes (FHPs). This model uses a limited number of experimental data for calibration with two parameters, βexp,1 and βexp,2, which account for uncertainties in the accommodation coefficients used to determine evaporation and condensation mass fluxes. This calibration, which is a key feature of the developed model, addresses the uncertainties and challenges in determining these coefficients. Moreover, the model incorporates the effects of interfacial shear stress, axial wall conduction, heat transfer in the liquid block region, and the contact angle of the liquid–vapor interface. Furthermore, the evaporation mass flux from the curved liquid–vapor interface is calculated using a separate multiscale approach developed in two recent works for analyzing evaporation from the extended meniscus in microchannels, considering thin-film evaporation. The model demonstrates high accuracy in predicting the maximum heat transport capacity (Qmax) and wall temperature distribution across various input heat powers, with a maximum error of less than 0.5 % in wall temperature predictions, as validated against experimental data. The validated model is then used to explore a new micro-grooved wick structure featuring converging/diverging microchannels. In this structure, the channel width is constant and equal to Wf2 in the first half of the heat pipe, from the condenser end to the midpoint, and then it decreases or increases to Wf1 by the end of the evaporator section. Results indicate that a flat heat pipe with a converging micro-grooved wick structure (Wf1 = 200 µm and Wf2 = 160 µm) can enhance Qmax from 53 to 70 W, 70.8 to 91.8 W, and 91.2 to 116.5 W at working temperatures of 60 °C, 70 °C, and 80 °C, respectively. This means the new converging micro-grooved wick structure can boost flat heat pipe performance by more than 25 % compared to a flat heat pipe with a straight micro-grooved wick (Wf1 = Wf2 = 200 µm) without increasing the occupied space.

Effect of pin fins on heat transfer during condensation in minichannel heat exchanger

In this study, condensation heat transfer of water vapor in a minichannel heat exchanger on smooth and pin fins surfaces was investigated experimentally. The experimental study was carried out to evaluate heat transfer coefficient, overall heat transfer coefficient, and Nusselt number over different ranges of vapor mass flux from 0.0064 to 0.0368 kg m−2 s−1 and cold-water flow rate range between 0.0013 kg s−1 to 0.0057 kg s−1. The minichannel has a rectangular shape with a hydraulic diameter of 1.3 mm. The experimental testing is carried out on aluminum surface with a channel that has a length of 270, width of 30 mm, and height of 1.3 mm. The pin fins surface is on the bottom of the condensing channel and the fins are circular and have diameter, height, and spacing of 1 mm, and are in-inline arrangement. It is found that condensation heat transfer coefficient on pin fins surface is about 15% higher than that on smooth surface. It is found that the condensation heat transfer coefficient increases significantly with increasing the vapor mass flux. In addition, the effect of increasing the cold fluid flow rate is lower than that of the hot vapor flow rate, but it becomes more significant for higher vapor flux.

On the influence of heat-induced evaporation over interfacial atomization driven by surface acoustic waves

In this work, the influence of evaporative flux over the atomization driven by surface acoustic waves (SAWs) of a Newtonian water drop is studied. The drop is placed on a heated substrate at constant temperature, higher than the saturation temperature at a given vapor pressure. In this manner, an interfacial temperature distribution arises along the drop free surface in terms evaporative mass flux and vapor recoil, which repercussion over aerosol size is studied by determining the asymptotic evolution equation governing the acoustically driven free surface. At such scenario, the connection between surface tension and temperature is also considered; thus, thermocapillary flow is incorporated into our drop model, described in terms of fundamental parameters, like the evaporation number, Marangoni number, and acoustic capillary number. Numerical solution of the evolution equation led us to obtain a simplified representation of the drop interfacial deformation mechanism, capable of predicting atomization and portraying the influence of evaporation over atomization. Subsequent analysis shows that the incorporation of evaporation at SAW atomization traduces in normal stresses counteracting the acoustic and thermocapillary effect, leading to the development of smaller drop aspect ratios with respect to the no-evaporative case. Being aware that the aerosol size is deeply related to the aspect ratio, we propose an analytical expression to estimate aerosol diameter under evaporative conditions. The results show that aspect ratio reduction leads to a decrement on aerosol size, up to two orders of magnitude, with respect to the no-evaporative case. Our study is a first approach providing insight about the importance of evaporation on aerosol regulation at SAW atomization.

Explicitly defined empirical constant for phase-change simulation in a two-phase closed thermosyphon

Over several decades, the importance of energy recovery has grown because of the global shift from fossil fuels to renewable energy sources. Among various energy-recovery approaches, waste heat recovery has emerged as a prominent method owing to its ease of adoption in various fields such as power plants and commercial buildings and high amount of recovery by adopting in the heavy industry. A heat exchanger with a heat pipe offers superior waste heat recovery efficiency through its highly effective heat transfer in a two-phase closed thermosyphon (TPCT). Numerous investigations have explored the thermal hydraulic performance of TPCTs in various geometries and operation conditions, mostly relying on experimental methods owing to the challenges in accurately simulating phase change in TPCT systems. These challenges arise from using the mass-transfer intensity factor, r—an empirically selected constant in phase-change models—which lacks a robust basis in fundamental principles. This study introduces a novel approach to selecting the r value for simulating the phase-change process in TPCTs. By employing the volume-of-fluid model for multiphase interfaces along with the widely used Lee model for phase change, the proposed model establishes a correlation between evaporation and condensation mass fluxes to define the r value in condensation heat transfer within a TPCT. The utilisation of the proposed model in simulations yielded improved temperature predictions, deviating by 4–7 K from experimental results while offering a significant reduction in the number of r values to choose from, achieved by explicitly defining the r value for condensation heat transfer. The conventional approach was to set both empirical constants to re = rc = 0.1 s−1. However, this model, which provides a relationship between the two empirical constants based on mass balance, revealed that rc should be 9.8 s−1 rather than 0.1 s−1 corresponding to re = 0.1 s−1. Furthermore, the study examines the diverse thermal performance outcomes associated with varying r values for evaporation in a wide range of re (0.1–100 s−1). rc was in the range of 9.8–12.7 s−1, denoting an increase of only 30% as re increased 1000 times from 0.1 to 100 s−1. This research offers a comprehensive understanding of the computational modelling of TPCTs, providing valuable insights into their thermal characteristics based on the intricate phase-change phenomena occurring within them.

Comparative analysis of entropy generation in smooth and micro-fin tubes using R513A refrigerant: A parametric study

Currently, although significant advancements have been made in understanding heat transfer coefficients and pressure decreases in different tube shapes, there is still a noticeable lack of detailed studies on the generation of entropy under flow boiling conditions. In this work, the entropy generation in micro-fin tubes (MFT1 and MFT2) and smooth tubes (ST1 and ST2) in flow boiling conditions experimentally investigated with refrigerant R513A. Research focused on evaluating influence of different input parameters on entropy generation, specifically contribution of heat transfer coefficient (HTC) and total pressure drop (TPD) on entropy generation for all testing tubes. As at a heat flux of 6 kW·m−² and a saturation temperature of 12 °C, MFT1 shows HTC contributions to entropy generation ranging from 0.032 to 0.156 W·K−1, while MFT2 ranges from 0.03 to 0.14 W·K−1. TPD contributions for both MFT1 and MFT2 range from 0.001 to 0.04 W·K−1. Hence, MFT2 shows better results than MFT1 as low entropy generation required for a good heat exchanger. Among input parameters, heat flux displays the highest sensitivity, indicating its significant influence on total entropy generation variations, while vapor quality, mass flux, and saturation temperature also demonstrate notable sensitivity. This research helps us design systems that transfer heat more effectively while using less energy. Understanding these factors can lead to more efficient heat exchangers and other thermal systems.

A novel phase-field lattice Boltzmann framework for diffusion-driven multiphase evaporation

Heat transfer and phase change phenomena, particularly diffusion-driven droplet evaporation, play pivotal roles in various industrial applications and natural processes. Despite advancements in computational fluid dynamics, modeling multiphase flows with large density ratios remains challenging. In this study, we developed a robust and stable conservative Allen–Cahn-based phase-field lattice Boltzmann method to solve the flow field equations. This method is coupled with the finite difference discretization of vapor species transport equation and the energy equation. The coupling between the vapor concentration and temperature field at the interface is modeled by the well-known Clausius–Clapeyron correlation. Our approach is capable of simulations under real physical conditions and is compatible with graphics processing unit architecture, making it ideal for large-scale industrial simulations. Three validation test cases are conducted to demonstrate the consistency of the presented model, including simulations of Stefan flow, the evaporation of suspended droplets containing water, acetone, and ethanol in the air, and the evaporation of a water sessile droplet on a flat surface. The results show that the model is able to predict the behavior and characteristics of each case accurately. Notably, our numerical results exhibit a maximum relative error of approximately 1% in simulations of Stefan flow. In the case of suspended droplet evaporation, the observed maximum difference between the calculated wet bulb temperatures and those derived from psychrometric charts is approximately 0.9 K. Moreover, our analysis of the sessile droplet reveals a good agreement between the results obtained by our model for the evaporative mass flux and those obtained from the existing models in the literature for different contact angles.

Numerical investigation on dynamic flow characteristics of methane condensation in microchannels

Microchannel condensers play an essential role in cryogenic two-phase heat management systems due to their efficient heat transfer characteristics. Thus, it is worth conducting an in-depth study on microscale condensation characteristics of cryogenic fluids. This paper delves into the flow condensation process of methane in microchannels. A two-dimensional transient model with high accuracy for cryogenic fluids has been developed by combining a self-defined program for the source term of the phase transition model. The model fully considers the boundary layer thickness and accurately explores the mesh accuracy. The complete condensation flow patterns are captured for various vapor quality, mass flux, and wall subcooling degrees. The injection flow is a unique flow regime for condensation in microchannels. The decrease in wall subcooling degree and increase in mass flux leads to the separation point at the neck of the injected flow moving towards the exit, while the annular flow region is expanding and the flow pattern transition is lagging. The mass flux improves the heat transfer coefficient more significantly at high vapor quality. During injection and bubble flow, the wall shear stress and local heat transfer coefficient are subject to bouncing and oscillations, which may induce fluctuations in the upstream annular flow. The prediction performance of six classical heat transfer correlations is evaluated. The results indicate that the Nie et al. correlation has the highest comprehensive prediction accuracy with MRD and MARD of -5.00 % and 15.83 %, respectively.

Mechanism investigation and model assessment of methane flow condensation in minichannels based on numerical simulation

Cryogenic refrigerants represented by methane differ significantly in thermophysical properties from working fluids at ambient temperature. Thus, examining their small-scale heat transfer and flow characteristics is essential for designing compact condensers within the cryogenic field. The numerical simulation of methane condensation in minichannels is conducted, and the process of phase-change mass and energy transfer is investigated by programming. Detailed condensation flow field information is obtained, and surface tension and gravity influences are elucidated. Synergy analysis indicates that the synergy near the tube wall still needs to be improved. Heat transfer performance is proved to be dependent on the relative significance of turbulence intensity and condensate film thickness. The tube inclination exerts a more noticeable influence on the condensation heat transfer for large diameters, which is supported by the dominance of gravity in the condensation heat transfer mechanism at larger diameters. At higher vapor quality and mass flux, the heat transfer enhancement governed by surface tension is more significant. The condensate at the bottom is mainly formed by the accumulation of condensate sliding off the tube top driven by gravity as the diameter increases, reducing the heat transfer region. The mass flux augments the frictional pressure drop more noticeably at high vapor quality. The prediction performance of empirical correlations is evaluated, and all the selected correlations underestimate the frictional pressure drop of methane. Moreover, the figure of merit analysis demonstrates that the pressure drop produced by diameter reduction is more substantial than heat transfer enhancement, suggesting the requirement to assess the pressure drop loss in practical applications.

Turbulent natural convection in an air–water system with evaporation across the free surface

In this paper we report on direct numerical simulations of turbulent convection in a cuboidal pool that contains liquid water in the lower part and air in the upper one. The bottom wall is uniformly heated and natural convection is established in both phases, accompanied by water evaporation across the free surface of the water. The descent of the free surface due to evaporation is computed via a newly developed tracking algorithm based on the ghost-fluid method. We present results for three different cases which correspond to different pool heights. In all of them, the natural convection in the water lies in the soft-turbulence regime. Whereas in the gas, it lies in the laminar, transitional and soft-turbulence regimes, respectively. Our analysis focuses on the characteristics of the convective patterns in the two phases and the statistics of the various flow quantities of interest. According to our simulations, the flow in the water is organized in a single-roll Large-Scale Circulation (LSC). In the gas, it is organized in single or dual-roll LSCs, depending on the aspect ratio of the pool. Interestingly, the impingement points of the LSCs of the two phases at the free surface remain very close to one another, which is attributed to the continuity of the shear stresses across the free surface. Further, after the initial transient period, both the free-surface temperature and the evaporative mass flux are stabilized and remain almost constant, but they exhibit small-scale fluctuations in time due to turbulence. Also, the transport of water vapor in air has similar properties as the heat transport, and the ratio between the Sherwood and Nusselt numbers is very close to the Lewis number for air.

Improving pressure drop predictions for R134a evaporation in corrugated vertical tubes using a machine learning technique trained with the Levenberg–Marquardt method

The present investigation utilized a machine learning structure to ascertain the pressure drop in vertically positioned, corrugated copper tubes during the evaporation process of R134a. The evaporator was a counter-flow heat exchanger, in which R134a flowed in the inner corrugated tube and hot water flowed in the smooth annulus. Different evaporation mass fluxes (195–406 kg m-2 s-1) and heat fluxes (10.16–66.61 kW m-2) were used with artificial neural networks at different corrugation depths. A multilayer perceptron artificial neural network model with 13 neurons in the hidden layer was proposed. Tan-Sig and Purelin transfer functions were used in the network model developed with the Levenberg–Marquardt training algorithm. The dataset, which consisted of 252 data points, related to the evaporation process, was divided into training (70%), validation (15%), and testing (15%) groups in an arbitrary manner. The artificial neural network model has been demonstrated to effectively forecast the pressure drop that occurs during evaporation. The mean squared error was computed for the ΔP values observed during the evaporation processes, yielding a value of 1.96E-03. The artificial neural network exhibited a high correlation coefficient value of 0.94479. The estimation fluctuations exhibited a range of ± 10%, whereas the experimental and anticipated ΔP data demonstrated a divergence of ± 10.3%.

Flow boiling heat transfer performance of R245fa in a vertical enhanced evaporator tube with sintering and electroplating composite coats

The R245fa flow boiling performance was investigated in a vertical evaporator tube of which inner wall surface is coated by a composite sintering and electroplating technology. The effects of vapor quality, heat flux, mass flux, and flow direction were compared and analyzed in the bare tube (BT) and the sintering and electroplating treated tube (SET). The results showed that the heat transfer coefficient increased and then decreased with the rise of vapor quality and heat flux. The heat transfer performance in downward flow was better than that in upward flow, while an opposite trend was observed for frictional pressure drop. With the increased of vapor quality, the frictional pressure drop increased and then decreased, but it monotonically increased with the increasing mass flux. The heat transfer enhancement mechanism was explored through flow pattern visualization. The annular flow pattern conversion line of SET was earlier than that of BT. Due to the strong wettability of SET, the thickness of liquid film was thinner in downward and the mother bubble was bigger in upward, which had a positive effect on flow boiling heat transfer. The maximum the heat transfer enhancement factor (EF) and the performance evaluation parameter (PEC) were 1.83 and 1.75, respectively.

Experimental investigation on the flow boiling of R134a in a plate heat exchanger with mini-wavy corrugations

As a common component in various applications, the two-phase heat transfer of plate heat exchanger has been frequently studied. However, further investigations are still required on the flow pattern, especially for those with new plate structures brought with the emergence of new applications. This study presented an experimental investigation on the flow boiling of R134a in a plate heat exchanger with mini-wavy corrugations designed for thermal management system of electric vehicle (EV). The flow pattern transitions in the test section were captured and discussed, meanwhile the effects of vapor quality and mass flux on the variation of heat transfer coefficient (HTC) were analyzed. Based on the observation, the two-phase flow pattern developed from pulsating film flow, then rough stream flow and finally to thin stream flow with the increase of vapor quality. The flow pattern map was determined with the superficial momentum of the liquid and vapor phase refrigerant, while the empirical correlations were developed based on two-phase Weber number to predict the transitions between the flow patterns. The HTC showed an intermediate increase firstly under low vapor qualities, and rapidly decreased after reaching the peak value at the vapor quality of 0.45–0.6. In addition, the experimental data were compared with the present correlations, and a new Nu correlation was developed considering the transitions of flow patterns, showing an optimal prediction on the experimental data with the mean absolute error of 11.3%.

Temperature depression model for cavitating flow with thermodynamic suppression effect in high-temperature water

The thermodynamic suppression effect of cavitation generally appears in cryogenic cavitating flows. Temperature depression, which is the temperature difference between the mainstream and inside the cavity, indicates the thermodynamic suppression effect. In this study, a temperature depression model is developed to understand the physical process of the thermodynamic suppression effect. The model is evaluated using the experimental data of temperature inside supercavitation in high-temperature water of up to 140 °C. The temperature depression model is derived based on Fruman's model and newly introducing the suppression effect of evaporative mass flux. At first, Fruman's model was compared with the experimental data. Fruman's model differed from the experimental data in the high-temperature region of more than 100 °C. In contrast, the proposed model reproduced the experimental data well in the high-temperature region. Therefore, introducing a suppression effect of evaporative mass flux is essential for describing the temperature depression in cavitating flow. In addition, the proposed model was expressed with existing parameters, and it was clear that the temperature depression was defined with the characteristic temperature of the B factor, the Nusselt number, and a term representing the suppression effect of evaporative mass flux expressed by dimensionless thermodynamic parameters.

EXPERIMENTS ON FLOW AND HEAT TRANSFER CHARACTERISTICS OF STEAM CONDENSATION IN SUBCOOLED WATER POOL

We report experiments of steam condensation in water pool for various steam mass flux (53-133 kg/ m<sup>2</sup>s) and water subcoolings (15-65°C). The process is visualized using high-speed imager, and the images are processed for bubble parameters and interfacial heat transfer. The bubble is observed to undergo a cyclic motion of growth, necking, and separation for the conditions studied. The flow transits towards chugging at higher subcoolings and low vapor mass flux. The heat transfer coefficient increases with vapor mass flux and liquid subcooling. The heat transfer coefficient is estimated to be 0.049-0.89 MW/m<sup>2</sup>K for oscillating regime, while relatively higher values (0.94-1.4 MW/m<sup>2</sup>K) are determined during chugging.

A multiscale analytical-numerical method for the coupled heat and mass transfer in the extended meniscus region considering thin-film evaporation in microchannels

A Computational Fluid Dynamics (CFD) framework is established to investigate the microscale transport phenomena in the evaporating extended meniscus region formed in rectangular microchannels. A wide range of microchannel widths (W = 1 to 250 µm) and wall superheats (ΔT = 0.01 to 5.0 K) are considered. Prior to performing CFD simulations, it is first necessary to employ thin-film evaporation modeling based on an augmented Young–Laplace model and kinetic-theory based model for the evaporating mass flux across curved surfaces to find the exact shape of the liquid-vapor interface along with disjoining and capillary pressure distributions. Two widely-used methods for the determination of boundary conditions (BCs) at the beginning of the thin-film region (i.e., setting δ0″ = 0 and finding δ0′ by the far-field BC or setting δ0′ = 0 and finding δ0″ by the far-field BC), where δ is film thickness, are thoroughly examined and the first approach is recommended because of its better convergence and ease of implementation in the thin-film evaporation model. Then, the two-dimensional domain of the extended meniscus is generated and discretized to simulate the evaporating liquid flow through the UDF programming in ANSYS Fluent. The developed framework is shown to be a simple yet powerful and practical method for accurately predicting the evaporation mass and heat fluxes from extended menisci in microchannels. The CFD simulation results indicate that the previous one-dimensional models which represent the state of the art in the literature in the field are not able to predict the rate of evaporative heat transfer from the extended meniscus in microchannels with an acceptable degree of accuracy. Based on the CFD simulations results, a multiple regression analysis is employed to establish several simple thermal resistance correlations. The correlations can be straightforwardly integrated into macroscale mathematical models of micro and miniature heat pipes for analyzing their thermal characteristics.

Experimental investigation on flow boiling heat transfer characteristics of R245fa/oil mixture in horizontal smooth tube

During the operation of the Organic Rankine Cycle (ORC) system, it's inevitable that the lubricating oil in the expander will mix with the working fluid, leading to a noticeable impact on its heat transfer performance. In this investigation, we conducted experimental research on the flow boiling heat transfer characteristics of a mixture of R245fa and POE (Polyolester) in a smooth horizontal tube. We explored the effects of various parameters, including oil concentration (ranging from 0% to 4%), heat flux (6 to 20 kW/m²), mass flux (ranging from 150 to 410 kg/(m²·s)), and vapor quality (ranging from 0 to 0.8). The results revealed that the heat transfer coefficient (HTC) for R245fa/POE increased with higher vapor quality and mass flux for each oil concentration. However, as the oil concentration increased, the rate of increase in HTC gradually diminished. Notably, an optimal oil concentration of 2.3% was identified to achieve a maximum HTC of 6 kW/(m²·K) in our experiments. Furthermore, our investigation into frictional pressure drop (FPD) indicated that higher vapor quality, mass flux, and heat flux all led to an increase in FPD for the R245fa/POE mixture. Additionally, we evaluated the impact of lubricants on the HTC and FPD of R245fa/POE mixtures using enhancement factor (EF) and penalty factor (PF), respectively. The EFs for HTC in the R245fa/POE mix ranged mainly between 0.95 and 1.6, while the PFs for FPD were concentrated between 0.75 and 1.4. The lubricating oil was found to enhance the flow boiling heat transfer performance of R245fa, with a comprehensive evaluation index range of 1.22 to 1.31. In terms of prediction accuracy, the Schlager-R22/150SUS correlation provided the most reliable estimates for the enhancement factor of HTC and the penalty factor of FPD, with mean absolute deviations (MAD) of 9.79% for EF and 14.03% for PF.

Critical Role of Boundary Conditions in Sorption Kinetics Measurements.

In order to characterize the hygroscopic properties of cellulose-based materials, which can absorb large amounts of water from vapor in ambient air, or the adsorption capacity of pollutants or molecules in various porous materials, it is common to rely on sorption-desorption dynamic tests. This consists of observing the mass variation over time when the sample is placed in contact with a fluid containing the elements to be absorbed or adsorbed. Here, we focus on the case of a hygroscopic material in contact with air at a relative humidity (RH) differing from that at which it has been prepared. We show that the vapor mass flux going out of the sample follows from the solution of a vapor convection-diffusion problem along the surface and is proportional to the difference between the RH of the air flux and that along the surface with a multiplicative factor (δ) depending only on the characteristics of the air flux and the geometry of the system, including the surface roughness. This factor may be determined from independent measurements in which the RH along the surface is known while keeping all other variables constant. Then we show that the apparent sorption or desorption kinetics critically depend on the competition between boundary conditions and transport through the material. For sufficiently low air flux intensities or small sample thicknesses, the moisture distribution in the sample remains uniform and evolves toward the equilibrium with a kinetics depending on the value of δ and the material thickness. For sufficiently high air flux intensities or large sample thicknesses, the moisture distribution is highly inhomogeneous, and the kinetics reflect the ability of water transport by diffusion through the material. We illustrate and validate this theoretical description on the basis of magnetic resonance imaging experiments on drying cellulose fiber stacks.

Flow condensation inside a multiport mini channel and a rectangular mini channel with pin fin array

In this paper, an experimental study of flow condensation was carried out of R134a in a multiport mini channel and a mini channel with pin fin array. The former comprised 20 parallel rectangular channels with an equivalent diameter of 0.64 mm. The latter is a narrow rectangular mini channel containing 10 rows of diamond pin fins with a staggered configuration, and height and longitudinal/transverse pitch of 0.5 mm and 2 mm respectively. To acquire the local value of heat flux and heat transfer coefficient, air jet impingement cooling devices were adopted to condense the vapor of refrigerants. The effects of vapor quality, mass flux, heat flux, and saturation pressure on flow condensation heat transfer coefficient were investigated with the operating conditions: vapor quality from 1 to 0, mass flux from 160 to 450 kg/(m2s), heat flux from 10.0 to 39.8 kW/m2, and saturation pressure from 600 to 1500 kPa. The experimental results indicated that the pin fin array significantly improves the condensation heat transfer coefficient up to nearly 186 % compared to the multiport mini channel in the high vapor quality region. For both channels, the local heat transfer coefficient increases with an increase in vapor quality, mass flux, and heat flux whereas decreases with increase in saturation pressure. The influence of heat flux and mass flux on heat transfer coefficient was more pronounced in the high vapor quality region than in the low vapor quality region. A sensitivity analysis was performed at different vapor quality levels. The most influential parameters in the smooth channel and the pin fin array are saturation pressure and mass flux, respectively. The relative contribution of heat flux is more obvious in the high vapor quality region. Some of the existing correlations have good prediction performance for the smooth channel experimental data in this paper, and the corresponding MAD is within 20 %. However, their deviations are much greater for the applications of pin fin array.

Multi-objective optimizations of vapor-liquid adjustment evaporator and its machine-learning based operational control strategy

During flow boiling, there exists highly efficient heat transfer peaked at the high vapor quality. The vapor-liquid adjustment evaporator employs the liquid drainage and liquid refilling to redistribute the vapor quality and mass flux. In this way, the efficient heat transfer could be repeated, leading to the improved heat transfer capacity and reduced pressure drop at the same time. However, the path arrangement and separation efficiencies have been not mutually coordinated to release the potential of the vapor-liquid adjustment evaporator at various conditions. In this study, a numerical model of this evaporator is developed and verified by experimental data. By implementing the multi-objective optimization algorithm, three optimal layouts, targeting to the lowest pressure drop, the highest heat transfer capacity and the compromised one, are obtained at the design conditions. Comparisons of their local characteristics reveals that the fifth path offers most benefits in terms of 50 % entire heat transfer capacity and up to 73 % reduced pressure drop. At various off-design conditions, the constant separation efficiencies in vapor-liquid adjustment evaporator could lead to the inferior performance to the conventional evaporator. By implementing the machine-learning based control strategy, it could have maximumly 5.2 %-10 % increased heat transfer capacity or 5.2 %-51 % reduced pressure drop.

Entropy generation and exergy loss of R123-MWCNTs nanorefrigerant in flow condensation

This experimental study examines the entropy generation due to heat transfer and frictional pressure and the exergy destruction during the condensation of an R123-MWCNTs nano-refrigerant within a horizontal tube. The effects of entropy production, Bejan number, and exergy loss are studied, together with the roles played by vapour quality, mass flux, and multi-wall carbon nanotubes (MWCNTs) concentration. The results show that the heat transfer entropy generation reduces as the vapour quality or mass flux and concentration of MWCNTs rises. In contrast, the entropy production due to frictional pressure during condensation increases with the rise in mass flux, vapour quality, and the MWCNTs fractions. The reductions in heat transfer entropy generation are high compared to the increment in frictional entropy generation, thus reducing the total entropy generation of the system. Further, nanofluid minimizes the system's Bejan number and exergy loss. The highest reduction in entropy generation and exergy loss of about 30 % and 29 %, respectively, was observed using the proposed nanofluid at a MWCNTs fraction of 0.1 % and mass velocity of 400 kg/m2s.