Simultaneous Heat Transfer Research Articles

Effect of Different Drying Systems on Drying Performance of Maraş Green Pepper (C.annum)

Drying is the simultaneous transfer of heat and mass, which is defined as the reduction of moisture in food. The aim of the study the drying performances of refractance window drying (95°C), fluidized bed drying (95°C, 2m3/m air velocity), and convective drying (95°C) were examined in the drying of Maraş green pepper (C.annuum). Drying performance was evaluated for effective diffusion coefficient (Deff), activation energy (Ea), Chroma (C) and total color change (ΔE). Drying curves were obtained by recording sample weights in 10-min periods. For the refractance window drying, fluidized bed drying and convective drying the time for the samples to reach 6-7% humidity level according to the wet base was found to be 70, 80 and 110min, and the effective diffusion coefficient was 6.49x10-10, 5.68x10-10 and 4.87x10-10 m2/s the activation energy was 53.54, 54.65 and 55.93kJ/mol, respectively. When the color properties are examined the Chroma value was determined as 18.23, 8.85 and 4.80 and the total color as 15.42, 26.29 and 30.33, respectively. It was seen that the closest value to the fresh product was in the samples dried with a refractance window drying. In the study, it was concluded that the use of a refractance window drying shortened the drying time by 14-36%, increased the effective diffusion coefficient, provided drying with lower activation energy, and better preserved the color quality in the production of dried Maraş green pepper.

A regression-based approach for the explicit modeling of simultaneous heat and mass transfer of air-to-refrigerant microchannel heat exchangers

With the increasing utilization of mathematical models for heat exchanger optimization studies, there lies the demand for robust and accurate yet fast modeling methodologies on the simultaneous heat and mass transfer process. This research conducted an in-depth review of mathematical models for heat exchanger performance prediction. We categorized typical modeling methodologies into lumped models, zone models, and numerical methods. The review of mathematical models revealed the iterative approaches for performance prediction of fully wet conditions are computationally expensive. Also, the assumptions and derivations based on adiabatic fin tips do not allow flexibility with advanced fin designs. In order to address these two issues, this research developed a systematic regression approach under a common range of geometrical and thermal parameters under both wet and dry conditions, single-phase and two-phase refrigerant flow. Such regression analysis provides an explicit calculation procedure for micro-channel heat exchangers' performance prediction. The improved semi-regression models show advances in accuracy and calculation speed within the selected ranges of geometries and working conditions for heat pump applications.

Pore Evolution in Cell Walls of Food Tissue during Microwave-Assisted Drying: An In-Depth Investigation.

Microwave (MW) heating is a unique approach that, unlike conduction- and convection-based heating, can provide volumetric heating. Complex microstructural changes in food materials occur because of simultaneous heat and mass transfer during drying, significantly affecting food structure and quality. Food properties, drying methods, and other drying parameters all have an impact on the microstructure of food samples, which in turn affects drying kinetics and food quality. However, no study has been undertaken to investigate the development of nano-micro-pores (NM-pores) on the cell walls and their relationship with the moisture migration mechanism. This study presents a novel investigation of the microstructural changes in food during microwave drying, with a focus on the formation of nano-micro-pores (NM-pores) on cell walls and their impact on moisture transport kinetics. The utilized hot air was maintained at a temperature of 70 °C, whereas microwave (MW) power levels of 100 W, 200 W, 300 W, and 400 W were used in microwave drying. The findings of the study indicate that the development of NM-pores occurs only during intermittent microwave drying (IMCD), while the cell wall of the food samples tends to burn or collapse in continuous microwave drying (CMD) due to the high heat generated. Additionally, no NM-pores were observed in the cell wall during convective drying. During IMCD with microwave power ranging from 100 W to 400 W, a range of pore sizes from 0.1 μm to 8.5 μm were observed. Due to the formation of NM-pores and collapses, MW drying takes around 10-20 times less time than convective drying to remove the same quantity of moisture. The effective moisture diffusivity values were found to be the highest in CMD at 4.70 × 10-07 m2/s and the lowest in CD at 2.43 × 10-09 m2/s. IMCD showed a moderate diffusivity of 2.45 × 10-08 m2/s. This study investigates the formation of NM-pores on cell walls during microwave drying and their impact on moisture transport kinetics and establishes correlations between microstructure modifications and moisture migration pathways.

Development of a method for predicting the transient behavior of an absorption chiller using artificial intelligence methods

Absorption chillers are complex equipment compared to mechanical chillers due to their nature of heat and mass transfer processes, working fluids used, and their limitations, such as crystallization problems and dealing with vacuum pressures. Even more, when their dynamic behavior is conducted, it leads to a challenging study due to the simultaneous heat and mass transfer present in the thermal compressor. In line with this, the need to know or measure the values of internal parameters, such as temperature, concentration, and internal flows. To perform the measuring leads to using complex physical models to simulate the transient behavior, making it hard to adapt more efficient control strategies. Hence, this work proposes a methodological strategy to determine the transient behavior of a LiBr/H2O single-effect absorption chiller through intelligent regression methods. The methodology was developed by analyzing external data from an absorption refrigeration system's hot, cold, and chilled water circuits, characterizing regression models exclusively with temperature inputs, and using four machine learning methods, such as linear regression, decision tree, random forest, and artificial neural network. One novelty of the work is the integration of the first law of thermodynamics and the heat exchanger characteristic equation with machine learning methods, such as linear regression (LR), decision tree (DT), random forest (RF), and artificial neural network (NN) to estimate the dynamic of absorption chillers through exclusively external data such as temperature and flow, and nominal data of the heat exchangers. Another novelty is the capacity to determine the internal conditions of the equipment, considering a full or partial operation. The validation and calibration procedure established that all the methods applied (LR, DT, RF, and NN) presented a good fit showing values above 0.95 of the correlation coefficient R2 and RMSE values less than 0.1. The results showed that the strategy allows for estimating the transient behavior of the outlet temperature profiles of the hot, cold, and chilled water circuits. The behavior parameters were calculated with reasonable accuracy showing errors of less than 1 % for all methods applied. The thermal COP value estimated was approximately 0.71 ± 0.05, within the values presented by the equipment manufacturer. Regarding the results, the proposed strategy represents a robust and accurate dynamic analysis tool for absorption chillers operating at full or partial load.

Numerical Simulation of the 3D Simultaneous Heat and Mass Transfer in a Forced Convection Solar Drying System Integrated With Thermal Storage Material

Abstract The demand for quality dried products necessitates cost effective and innovative drying techniques that will improve its market value. The slow drying rate, weather dependency, and moisture reabsorption have been identified as the major challenges of solar drying operation. To address these shortcomings, hybrid solar drying systems have been recommended for the drying of various agricultural materials and other porous products. Designing a better drying system to accommodate thermal storage materials requires detailed analysis, which could be achieved through numerical simulation. Therefore, the numerical simulation of heat and mass transfer in a forced convection solar drying system integrated with black-coated firebrick sensible thermal storage materials (STSM) for the cocoa beans, locust beans, cereal grains, etc., was investigated under no-load conditions. The equations governing the fluid flow for a three-dimensional solar drying system were solved using the finite volume method with the aid of ansys, the computational fluid dynamics software to comprehend the dynamic and thermal behavior of the airflow within the dryer. The experimental maximum temperature values of 96.9 °C and 77.3 °C for the collector and drying chamber were in agreement with the simulated maximum collector and drying chamber temperatures of 116.9 °C and 80 °C respectively. The designed solar drying system with the incorporated STSM showed the capacity of raising the temperature of the air within the drying chamber to 3–37 °C above ambient temperature between 01:00 p.m. and 10:00 p.m. The agreement of the simulated dryer model with the experimental one is an indication that the developed dryer is suitable for drying cocoa, locust beans, fish, cereal grains, and some other agricultural products within an acceptable period based on the previous studies and therefore, the drying system is recommended to avoid the shortcomings associated with traditional/open sun drying.

Heat and Mass Transfer in Hygroscopic Porous Media: Consequences of Invoking Tóth’s Sorption Isotherm

The thermal performance of ventilated beds of hygroscopic porous media depends strongly on the sorption properties of the solid phase. Furthermore, the published literature shows that the predicted performance of ventilated beds of hygroscopic porous media is extremely sensitive to the form and accuracy of the sorption isotherm. This is particularly apparent under idealized conditions when thermodynamic equilibrium is deemed to exist between the solid and fluid phases. When Tóth’s isotherm is invoked in an equilibrium model used to analyse the performance of a bed of initially warm and dry silica gel ventilated with cool, humid air, it is predicted that a shock wave develops downstream of air entering the bed. In contrast, it is shown that non-equilibrium solutions result in a transfer wave, the width of which decreases as the velocity of the interstitial air decreases. The Tóth isotherm gives rise to erroneous values of the integral heat of wetting of silica gel. Previously published studies of simultaneous heat and mass transfer in beds of hygroscopic materials refer to conceptual difficulties in interpreting the results, and some published mathematical analyses are somewhat terse. A secondary aim of this work is to assuage these difficulties by detailing the analyses in Supplementary Information.

Semiempirical modeling of a traditional wood-fired pizza oven in quasi-steady-state operating conditions.

Wood-fired ovens are mandatorily used to bake the Neapolitan pizza. Unfortunately, they are still empirically operated. In this work, a pilot-scale wood-fired oven was kept operating in quasi-steady-state conditions. Once the combustion reaction of oak logs had been modeled, the composition of flue gas measured, and the external oven wall and floor temperatures thermographically scanned, it was possible to check for the material and energy balances and, thus, assess that the heat loss rates through flue gas and insulated oven chamber were, respectively, equal to 46% and 26% of the energy supplied by burning firewood. The enthalpy accumulation rate in the internal fire brick oven chamber amounted to about 3.4kW, this being adequate to keep not only the temperatures of the oven vault and floor practically constant but also to bake one or two pizzas at the same time. Such a rate was predicted by contemplating the simultaneous heat transfer mechanisms of radiation and convection between the oven vault and floor surface areas. The efficacy of the semiempirical modeling developed here was further tested by reconstructing quite accurately the time course of water heating in aluminum trays with a diameter near to that of a typical Neapolitan pizza. The heat flow from the oven vault to the water-containing tray was of the radiative and convective types for about 73% and 15%, whereas the residual 12% was of the conductive type from the oven floor. PRACTICAL APPLICATION: Despite wood-fired ovens are largely used in the restaurant and food service industry, their operation is highly dependent on the operator's ability. This study shows how the heat loss rates through flue gas and insulated oven chamber can be assessed, and how the enthalpy accumulation rate in the internal fire brick oven chamber can be predicted by accounting for the simultaneous heat transfer mechanisms of radiation and convection between the oven vault and floor surface areas. The efficacy of this semi-empirical modelling was further checked for via some water heating tests.

Recoil and solidification of a paraffin droplet impacted on a metal substrate: Numerical study and experimental verification

Drop impact is an important phenomenon in many industrial processes, such as ink-jet printing, plasma spraying, and spraying cooling. This study focused on numerical prediction of the whole drop-impact process, including impact, flattening, recoil, and solidification of the droplet, based on the finite difference method. A volume-of-fluid method was used to track the free surface. Drop impacts under both isothermal and non-isothermal conditions were investigated with consideration of the effects of the droplet temperature and impact velocity. Drop-impact videos were recorded with a high-speed camera to verify the simulation results. The simulation results agreed well with the experimental results of the spreading factor, recoil height, and geometry of the solidified splat. The numerical and experimental results showed that cooling and solidification of the droplet due to simultaneous heat transfer from the droplet to the substrate significantly reduced the spreading factor and facilitated recoil of the impacted droplet. In addition, a smaller spreading factor, a larger recoil height, and a thicker solidified splat occurred for lower droplet temperature and lower impact velocity.

Preparation of cellulose/chitosan superoleophobic aerogel with cellular pores for oil/water separation

Nanofibrillated cellulose (NFC)/chitosan (CS) composite aerogel with underwater superoleophobicity was prepared by freeze-drying for oil/water separation. The soft-ice freezing technique was employed to construct cellular pores for improving the mechanical performance while enhancing mass transfer of freeze-drying. The dielectric-aided microwave heating was adopted to enhance heat transfer of freeze-drying. The results showed that the prepared NFC/CS aerogel exhibited an excellent mechanical property with a stress of 87.16 kPa at 80% strain, and underwater superoleophobicity with all oil contact angles above 150˚. The proposed drying method could save over 40% of drying time, achieving a simultaneous mass and heat transfer enhancement of freeze-drying. The prepared aerogel provided separation efficiencies of all above 99.96% and average permeate fluxes of 0.817 and 0.826 L·m−2·s−1 after 20 cycles for oil/water and oil/seawater separations. The research results are expected to provide an economical and sustainable solution for the industrial oily wastewater treatment.

Study on efficient and accurate protocols of measuring sorption isotherm of porous building materials using three-dimensional hygrothermal simulation

The sorption property of porous building materials in the hygroscopic region (sorption isotherm) is an important input to hygrothermal simulation. In this study, we proposed efficient and accurate protocols to obtain sorption isotherms using the static desiccator method. We developed a calculation model for three-dimensional simultaneous heat and moisture transfer in a material corresponding to the measurement specified in ISO 12571. Using an international database of the material properties, we conducted numerical simulations of the sorption process for five types of materials. The evolution of the amount of adsorbed moisture in a specimen and the time to reach equilibrium during the measurement were calculated under various humidity levels and specimen dimensions. According to the simulated results, we improved timing and interval for weighting the specimen to confirm the attainment of equilibrium for each type of a material and for each humidity level from the viewpoint of efficiency and accuracy. Meanwhile, the influence of size of a specimen on the time necessary to obtain the results was quantitatively demonstrated. Moreover, it was demonstrated that the method used for the determination of the attainment of equilibrium provided in ISO 12571 can underestimate the moisture content of materials with low moisture content and vapor permeability, and a policy to reduce risks of the underestimation was proposed.

Numerical study of gyrotactic bioconvection in stretching flow of a variable viscosity nanofluid with buoyancy and power-law wall effects

Modern aerospace is increasingly embracing bio-inspired designs. Intelligent coatings are being developed which amalgamate nanotechnological and also biological features including, bio-rheology, anti-corrosion/fouling features and strategic embedded of micro-organisms which respond to specific stimuli including light, heat, chemical concentration, electrical or magnetic fields. This article develops a mathematical model for steady, 2-D, incompressible laminar boundary layer flow and gyrotactic bioconvection in a variable viscosity nanofluid along a convectively heated stretched plate. The model is driven by the investigation of innovative bio-inspired nanocoatings. The behavior of nanoparticles and microorganisms, which is non-isothermal and non-isosolutal, has been taken into account. The Buongiorno two-component nanoscale model is employed which permits simultaneous mass and heat (nano-particle) transfer. A boundary layer framework is deployed. The transformed boundary value problem is solved numerically by using finite difference method with central differencing, tridiagonal matrix manipulation and Iterative algebraic solution procedure. Validation with former studies for selected special cases of the generalized model is conducted. Further validation of the general model solutions is achieved with a generalized differential quadrature (GDQ) algorithm. The major inspections of the current work are the flow is accelerated with greater viscosity parameter and bioconvection Rayleigh number. Future investigations will explore more complex non-Newtonian constitutive models.

Performance Evaluation of Low-cost Grain Dryer

Paddy is India’s most significant agricultural crop, accounting for more than 40% of the nation’s overall production of food grains. The moisture content of 10.0 to 12.0 %, which is suitable for safe storage, must be achieved after the drying of paddy. Drying of materials having high moisture content is complicated process, involving simultaneous heat and mass transfer. In this study the performance of grain dryer was evaluated for paddy grains for two different paddy varieties, Rajshree weighed 372 kg and Rajendra Kasturi weighed 366 kg with Initial Moisture Content IMC of Rajshree 14.00 % w.b. (V1) and of Rajendra Kasturi 13.40 % w.b. (V2). Initial Moisture Content (all in w.b.) of variety V1 was 14.00 % and of variety V2 was 13.40 %. It was reduced to the Final Moisture Content (all in w.b.) of 10.10 % for variety V1 and 9.60 % for variety V2. It was found that 1000 grains weight of paddy grains was reduced due to loss of moisture. It is also observed that average length and average breadth of grains decreases with decrease in moisture content for both the varieties of paddy grains. It was found that the bulk density of paddy grains after drying was 578.9 kg/m 3 and 555.58 kg/m3 V1 and V2. Decrease in bulk density is natural phenomenon during drying which was also observed here. It was observed that percent moisture reduction was lower at initial stage of drying for variety V1, which became higher at later stage of drying. However, the percent moisture reduction was higher at initial stage of drying for variety V2, which became lower at later stage of drying. Paddy was dried up to 10.10 % and 9.60 % for variety V1 and V2 respectively during this time. The drying efficiency for both varieties V1 and V2 was found as 35.36 % and 36.85 % respectively. The heat utilization factor and coefficient of performance was found as 0.948 and 0.052 for variety V1 and 0.960 and 0.040 for V2. Grain dryer is capable of drying paddy grains within reasonable time. Temperature and moisture profiles during paddy drying are found well accepted for drying of paddy. The quality of dried paddy grains was reasonably good and acceptable. Considering the level of technology and the capacity, the dryer meets the requirements of paddy traders and farmers of Bihar.

Different Forms of the Correction Factor Used to Describe Simultaneous Heat and Mass Transfer

Different Forms of the Correction Factor Used to Describe Simultaneous Heat and Mass Transfer

Energy and exergy analysis of a novel direct-expansion ice thermal storage system based on three-fluid heat exchanger module

Direct-expansion ice thermal storage (DX-ITS) system can overcome the mismatch between cold energy supply and demand, and also exhibit the characteristics of high energy efficiency, simple pipeline and minimal installation space. However, existing DX-ITS failed to achieve simultaneous high-efficiency heat transfer in the processes of charging and discharging, thereby limiting its widespread use. This study developed a novel DX-ITS system based on three-fluid heat exchanger modules employing micro heat pipe arrays to address the proceeding issue. Temperature distribution, pressure, charging and discharging power, energy efficiency ratio, exergy efficiency and ice packing factor during the charging and discharging processes at different compressor speeds, cooling-air temperature and flow rates were experimentally studied. Results demonstrated that maximum charging power, energy efficiency ratio, exergy efficiency, ice packing factor and discharging power of proposed system using a 4 HP compressor under experimental conditions reached 5.71 kW, 2.31, 11.12 %, 82.1 % and 4.99 kW, respectively. Moreover, cooling-air temperature had a more significant effect on system performance compared with cooling-air flow rate and compressor speed. The 11.7 %, 25.9 %, 24.3 %, and 20.6 % reductions in charging power, energy efficiency ratio, exergy efficiency, and ice packing factor were observed with an increase of cooling-air temperature from 18 °C to 35.5 °C.

MHD convective rotating flow of viscoelastic fluid past an infinite vertical oscillating porous plate with Hall effects

AbstractIt is considered the unsteady and incompressible magnetohydrodynamic rotating free convection flow of viscoelastic fluid with simultaneous heat and mass transfer near an infinite vertical oscillating porous plate under the influence of uniform transverse magnetic field and taking Hall current into account. The governing equations of the flow field are then solved by a regular perturbation method for a small elastic parameter. The expressions for the velocity, temperature, and concentration have been derived analytically and also its behavior is computationally discussed with reference to different flow parameters with the help of graphs. The skin friction on the boundary, the heat flux in terms of the Nusselt number, and the rate of mass transfer in terms of the Sherwood number are also obtained and their behavior discussed. The resultant velocity enhances with increasing Hall parameter and rotation parameter. The reversal behavior is observed with increasing viscoelastic parameters. The resultant velocity enhances and experiences retardation in the flow field with increasing radiation parameters, whereas the secondary velocity component increases with increasing rotation parameters. The temperature diminishes as the Prandtl number and/or the frequency of oscillations. The concentration reduces at all points of the flow field with the increase in the Schmidt number.

A generalized CFD model for evaluating catalytic separation process in structured porous materials

A hybrid multiphase model is developed to simulate the simultaneous momentum, heat and mass transfer and heterogeneous catalyzed reaction in structured catalytic porous materials. The approach relies on the combination of the volume of fluid (VOF) and Eulerian–Eulerian models, and several plug-in field functions. The VOF method is used to capture the gas–liquid interface motion, and the Eulerian–Eulerian framework solves the temperature and chemical species concentration equations for each phase. The self-defined field functions utilize a single-domain approach to overcome convergence difficulty when applying the hybrid multiphase for a multi-domain problem. The method is then applied to investigate selective removal of specific species in multicomponent reactive evaporation process. The results show that the coupling of catalytic reaction and interface species mass transfer at the phase interface is conditional, and the coupling of catalytic reaction and momentum transfer across fluid–porous interface significantly affects the conversion rate of reactants. Based on the numerical results, a strategy is proposed for matching solid catalyst with operating condition in catalytic distillation application.

Computational fluid dynamics simulations of solar-assisted, spacer-filled direct contact membrane distillation: Seeking performance improvement

Significant downstream performance reduction and concentration polarisation reduce direct contact membrane distillation (DCMD) efficiency. These challenges are not well researched since they are difficult to implement experimentally and numerically. Hence, this study examined the impact of solar absorbers and different spacer filaments upon DCMD performance. A 2D computational fluid dynamics model that considered simultaneous mass and heat transfer across the membrane and throughout the channels was developed to simulate water flux in DCMD modules under the use of solar absorbers and spacer filaments of various designs. The simulation outcomes were in excellent agreement with experimental results provided by two different studies, with the assisted solar absorber module and the spacer-filled module deviating <5 % and 3 % from the experimental results, respectively. A module equipped with the solar absorber membrane enhanced the DCMD performance better than a module with a solar absorber plate. The results also illustrated that a module with cylindrical detached spacer filaments improved DCMD performance more than a module with rectangular and semicircular attached spacers. Finally, it was shown that a module equipped with an integrated developed spacer and solar absorber membrane significantly enhanced water flux and both concentration and temperature polarisations, specifically when the inlet velocity was increased. Water flux and the temperature polarisation coefficient increased by 220 % in the integrated module, and the concentration polarisation coefficient decreased by 10 %. Moreover, the developed module resulted in a substantial increase in downstream performance.

Modeling and simulation of solvent behavior and temperature distribution within long stick propellants with large web thickness undergoing drying

Drying is a complicated physical process which involves simultaneous heat and mass transfer in the removal of solvents inside propellants. Inappropriate drying techniques may result in the formation of a hard skin layer near the surface to block the free access of most solvent through for long stick propellants with large web thickness, which lead to lower drying efficiency and worse drying quality. This study aims to gain a comprehensive understanding of drying process and clarify the mechanism of the blocked layer near the propellant surface. A new three-dimensional coupled heat and mass transfer (3D-CHMT) model was successfully developed under transient conditions. The drying experiment results show that the 3D-CHMT model could be applied to describe the drying process well since the relative error of the content of solvent between simulation and experiment values is only 5.5%. The solvent behavior simulation demonstrates that the mass transfer process can be divided into super-fast (SF) and subsequent minor-fast (MF) stages, and the SF stage is vital to the prevention of the blocked layer against the free access for solvent molecules inside propellant grains. The effective solvent diffusion coefficient (Deff) of the propellant surface initially increases from 3.4 × 10−6 to 5.3 × 10−6 m2/s as the temperature increases, and then decreases to 4.1 × 10−8 m2/s at 60–100 min. The value of Deff of surface between 0–1.4 mm has a unique trend of change compared with other regions, and it is much lower than that of the internal at 100 min under simulation conditions. Meanwhile, the temperature of the propellant surface increases rapidly at the SF stage (0–100 min) and then very slowly thereafter. Both the evolution of Deff and temperature distribution demonstrate that the blocked layer near the propellant surface has been formed in the time period of approximately 0–100 min and its thickness is about 1.4 mm. To mitigate the formation of blocked layer and improve its drying quality of finial propellant products effectively, it should be initially dried at lower drying temperature (30–40 °C) in 0–100 min and then dried at higher drying temperature (50–60 °C) to reduce drying time for later drying process in double base gun propellants. The present results can provide theoretical guidance for drying process and optimization of drying parameters for long stick propellants with large web thickness.

Heat and mass transfer in hygroscopic hydrogels

Sorption and desorption with hygroscopic hydrogels hold significant promise for thermal management, passive cooling, thermal energy storage, and atmospheric water harvesting. However, a comprehensive understanding of the energy and mass transport mechanisms in hygroscopic hydrogels remains missing, impeding accurate modeling and optimization. In this work, we develop a model for the simultaneous vapor, water, and heat transfer in hygroscopic hydrogels during sorption and desorption processes. We show that by considering vapor diffusion in the hydrogel micropores, water diffusion in the polymer mesh, and heat transfer in the porous hydrogel, we can accurately capture experimentally observed thermally-driven desorption rates in these hydrogels. Furthermore, we consider three typical operating configurations of hydrogels and elucidate the differences in the transport mechanisms depending on the configuration. Finally, for each of these configurations, we identify key design parameters, including hydrogel thickness, hydrogel shear modulus, heat transfer coefficient, and thermal conductivity, and we parametrically show that by varying these parameters, a hygroscopic hydrogel can desorb up to 128.5%, 14.9%, 69.7%, and 9.6% more water, respectively, relative to the initial water content. This work provides a generic framework to model sorption and desorption processes in hygroscopic hydrogels which can guide the design and optimization in applications of thermal management, passive cooling, thermal energy storage, and atmospheric water harvesting with hydrogels.

Film thickness, interfacial waviness and heat transfer during downflow condensation of R134a

• Condensation tests are performed with R134a in a 3.38 mm tube. • Simultaneous film thickness and heat transfer measurements are carried out. • The experimental results of R134a are compared to those of R245fa. • The interfacial waves inception is delayed for R134a due to the high vapor density. • Models for condensation heat transfer, film thickness and void fraction are assessed. Coupled measurements of liquid film thickness and heat transfer during condensation inside minichannels are rare in the literature due to the technical difficulties in performing accurate and non-intrusive measures. The availability of such experimental data is fundamental for better understanding the occurring physical mechanisms and for the proper design of heat exchangers. To cover this gap, in the present work liquid film thickness and heat transfer coefficients have been simultaneously measured during vertical downflow condensation inside a 3.38 mm inner diameter channel. Condensation tests have been run with refrigerant R134a at mass flux between 30 and 150 kg m −2 s −1 and 30 °C saturation temperature. The instantaneous liquid film thickness and interfacial waves are detected by combining two complementary optical methods: shadowgraph technique (applied to a sequence of flow pattern images recorded by a high speed camera) and chromatic confocal imaging. In the data analysis, the experimental results of R134a have been discussed in comparison to R245fa condensation data taken during a previous experimental campaign at much lower pressure. On average, R134a displays higher liquid film thickness than R245fa and the appearance of high-amplitude disturbance waves, which promote the thinning of the liquid film and enhance the heat transfer, is detected at higher mass flux for R134a as compared to R245fa. The liquid film thickness is found to decrease with the increasing mass flux for both fluids, except from 30 to 50 kg m −2 s −1 due to the transition between shear stress-driven and gravity-driven downflow condensation. The accuracy of several well-known models for the prediction of heat transfer coefficient, film thickness and void fraction has been assessed with comparison to the experimental data of both R134a and R245fa.