Convective Mass Transfer Coefficients Research Articles

Experimental and Statistical Determination of Convective Heat and Mass Transfer Coefficients for Eucalyptus nitens Sawn Wood Drying

This study aimed to develop a model using experimentally obtained convective heat and mass transfer coefficients to predict the effect of temperature, humidity, and drying rate on wood drying. Tangential wood samples of Eucalyptus nitens (H. Deane & Maiden) were used in the investigation. The experimental design consisted of two temperature levels (40 °C and 55 °C), two relative humidity levels (55% and 75%), and two air velocity settings (2 m·s−1 and 3 m·s−1). The experiments were conducted under a constant evaporation rate, spanning the maximum and critical moisture content in the wood. A statistical model using multivariate regression was created to predict the convective heat and mass transfer coefficients. The results indicated that the experimental data and empirical correlations exhibited an error margin of 37.77% and 37.86%, respectively. A significant positive correlation was found between the convective heat transfer coefficient and air velocity, temperature, and relative humidity, while the convective mass transfer coefficient showed a significant positive correlation only with air velocity and temperature. The model predicted the convective heat and mass transfer coefficients with high accuracy and statistical significance. Using the proposed method, we successfully obtained both convective coefficients, which enable accurate description of heat and mass flow during the convective drying of Eucalyptus nitens wood.

Bioethanol production from cassava fed-batch fermentation in pervaporation membrane bioreactor with permeate fractional condensation

A new type pervaporation membrane bioreactor was developed by coupling cassava-based fed-batch fermentation and pervaporation membrane separation technology for bioethanol production. Membrane separation slowed down broth accumulation rate and mitigated ethanol product inhibition. The fermentation time, average sugar consumption rate, ethanol production and ethanol productivity were 131h, 6.6 gL-1h-1, 149.4 gL-1 and 2.5 gL-1h-1, respectively. They were improved by 55.9 %, 73.2 %, 32.3 % and 49.7 %, respectively, compared to those of conventional fed-batch fermentation. The average membrane flux and separation factor were 711.3 gm-2h-1 and 6.5, respectively, with an ethanol concentration of 24.4 wt% in the permeate. Throughout fermentation, the total mass transfer coefficient remained constant at 9.6 × 10-6 ms-1, with the convection mass transfer coefficient decreasing as the broth viscosity increasing while the membrane mass transfer coefficient slightly increasing. A two-stage condensation downstream the membrane achieved fractional vapor recovery. The primary condensate with low ethanol concentration (1.6 wt%) can be used for cassava hydrolyzation, saving 14.0 % of water requirement. Recovering ethanol from the high-concentration (32.6 wt%) secondary condensate required lower energy (46 %) compared to direct recovery from the total permeate.

Energy analysis, mass transfer, and rehydration kinetics of okra dried using evacuated tube solar dryer

ABSTRACT Okra (Abelmoschus esculentus L.), like most fruits and vegetables, deteriorates quickly after harvesting due to its high moisture content and respiration rate. To preserve it and mitigate the post-harvest losses, a developed evacuated tube solar dryer (ETSD) with heat pipes was used to dry fresh okra. Thermal profiling of the ETSD was carried out to investigate the range of temperature gain inside the drying chamber. The effect of different blanching temperatures, viz. 70, 80, and 90°C, on drying characteristics and physical properties like color, water activity, and rehydration kinetics of okra were investigated. The energy efficiency of the collector ranged from 13.21% to 42.67%. Similarly, the convective mass transfer coefficient was calculated and ranged from 3.41 × 10−07 to 8.57 × 10−07 m/s. The okra blanched at 80°C, showing maximum greenness retention and lower water activity. The rehydration ratio was 6.05, 7.19, 7.73, and 7.41 for control, and okra blanched at 70, 80, and 90°C. The rehydration kinetics were analyzed using four distinct models: Peleg, Weibull, exponential, and first order. The Weibull model demonstrated the most accurate fit to the data. It is suggested that okra blanched at 80°C and dried under ETSD gives better retention of color and a higher mass transfer and rehydration rate.

Impact of ultrasound pretreatment on banana chip production by convective drying using hot air: Consideration of shrinkage and variable effective mass diffusivity in the process description

AbstractIn this study, we used experimental analysis and numerical simulation to describe and compare the convective drying (CD) of banana slices, without and with pretreatment with ultrasound (US) at 25 kHz. Banana slices, with an average initial thickness of about 6.60 × 10−3 m and an average diameter of about 3.01 × 10−2 m, were subjected to CD at temperatures of 60, 70, and 80°C, with air speed of 1.5 m s−1. Considering such dimensions, the one‐dimensional diffusion equation in Cartesian coordinates was numerically solved by the finite volume method, using a fully implicit formulation (direct problem). A solver with a numerical solution was developed for the direct problem. This solver was coupled to an optimizer to determine the parameters a and b of functions that relate the effective moisture diffusivity (D) with the local moisture ratio, and also to determine the convective mass transfer coefficient (h) (inverse problem), considering the shrinkage. The proposed model was used to describe the data obtained. The results revealed that the drying rate increased in samples that were subjected to pretreatment with US, for each drying temperature. The best‐simulated results were those that related D to the local moisture ratio by means of a bcosh (aMR2) function. It could be observed that pretreatment with US allowed saving of time (and consequently energy) of 28.3%, 16.0%, and 12.4% at drying temperatures of 60, 70, and 80°C, respectively.Practical applicationsThe main contribution of this paper is to present a solver and an optimizer to determine the mass transfer properties considering the shrinkage during convective drying of banana pieces pretreated with ultrasound by the inverse problem. With the information obtained for the mass transfer properties and the proposed numerical model with a hyperbolic cosine type function for the effective moisture diffusivity, it is possible to more accurately determine the time and, therefore, the energy savings of processing banana chips pre‐treated with ultrasound.

Effect of electrolyte concentration on bubble evolution and mass transfer characteristics on surface of photoelectrode

In the photoelectrochemical water splitting reaction system, bubbles will cover the reaction area on the photoelectrode surface, affecting the reaction impedance and gas-liquid mass transfer. A laser irradiation system is built and it is coupled with an electrochemical workstation and high-speed microscopic imaging system. The evolution behavior and mass transfer characteristics of single O<sub>2</sub> bubble on the TiO<sub>2</sub> photoelectrode are studied at different electrolyte concentrations (Na<sub>2</sub>SO<sub>4</sub>, 0.1–2.0 mol/L). With the increase of electrolyte concentration from 0.1 mol/L to 2.0 mol/L, the solution resistance and bubble additional resistance decrease, and the overpotential in the stable growth stage of bubble decreases from 0.113 V to –0.089 V. The bubble will cause the fluctuation of overpotential in the nucleation, growth and detachment stages, which is consistent with the impedance change caused by the change of dissolved oxygen concentration in the liquid phase. By analyzing the correlation between gas evolution efficiency and bubble coverage, it is found that the increase of electrolyte concentration will lead the bubble coverage and gas evolution efficiency to decrease simultaneously. By calculating the Sherwood dimensionless number, the results show that the total convective mass transfer coefficient increases with the electrolyte concentration increasing. Single-phase natural convection plays a dominant role in the process of gas product transfer, and its mass transfer coefficient is one order of magnitude larger than that of bubble-induced convection. In summary, by adjusting the electrolyte concentration, the bubble on the gas evolution photoelectrode surface can be effectively removed and the mass transfer of the system can be optimized, which is of great significance in improving the efficiency of photoelectrochemical water splitting.

A numerical study on autoignition of finite thick polymethyl methacrylate (PMMA) subjected to thermal radiation and forced airflow

A 2D numerical model coupling condensed and gas phases is developed to investigate autoignition of finite thick PMMA heated by radiation in forced airflow. Six sample thicknesses (1–15 mm), four heat fluxes (30–60 kW/m2), and five airflow velocities (0–1.2 m/s) are considered to examine their effects. The model is first verified by measured surface temperatures in experiments. Then three boundary layers (BL), velocity, thermal and concentration BLs, are studied based on simulation results and BL theory. BL thickness, local and surface-averaged frication coefficients, convective heat and mass transfer coefficients at solid–gas interface are estimated and compared with analytical results. Meanwhile, Damköhler numbers (Da) in non-ignition cases are calculated to reveal their impact. The results show that the model predicts measured surface temperatures well despite some divergence for thin samples due to deformation. Simulated velocities and thermal BL thicknesses agree well with analytical ones, yielding a maximum error of 33 % for concentration BL. Numerical convective heat and mass transfer coefficients match analytical ones well, whereas large discrepancy exists for friction coefficient. Two types of Da evolution, corresponding to different controlling mechanisms of non-ignition cases, separated by 6 mm sample are identified.

Computational fluid dynamics analysis of a micro-scale chamber for measuring organic chemical emission parameters

Computational fluid dynamics simulations are used to model the velocity field and the transport of a passive scalar within a micro-scale chamber used to measure diffusional transport through various building materials. Comparisons of solutions obtained using a steady, laminar flow assumption with velocity measurements obtained from hot-wire anemometry show that the numerical method generally underpredicts the near surface velocity field. The results improve for higher flow rates and for carpeted test materials, modeled as a porous resistive layer. Calculations involving scalar transport within the upper chamber of the sampling device are performed for different flow rates and Schmidt numbers. The results are used to develop a model for the convective mass transfer coefficient, correlated as a function of the Reynolds and Schmidt numbers as well as the porosity of the carpet. This model is integrated into a steady-state mass transport model for predicting the diffusion of gaseous formaldehyde through various test materials. Predictions of diffusion and partition coefficients for vinyl flooring, gypsum wall board, and carpet are within the ranges of literature data. The results indicate that a perfectly mixed upper part of the sampling device is an adequate assumption.

EFFECT OF ETHANOL IMMERSION AND ULTRASOUND PRETREATMENTS ON THE KINETICS OF CONVECTIVE DRYING OF QUINCE

This work studied the impact of ethanol immersion (E), ultrasound (U), and ultrasound-ethanol immersion (UE) for 10, and 20 min on the drying characteristics of quince. After pretreatments, the samples were dried at a hot dryer at 90℃ and air velocity of 2 m/s. Drying time was reduced by 50% for samples pretreated in Ultrasonic-Ethanol (UE20) for 20 min. Moisture diffusion coefficient varied from 1.880 to 2.933×10-9 m2/s. The friction drag force, convective heat transfer coefficient, and convective mass transfer coefficient were 6.110×10-6 N, 24.077 W/m2K, and 0.020 m/s, respectively. The thermal conductivity, the specific heat, and the density of quince samples ranged from 0.5278 to 0.5876 W/m. K, 3554.00 to 3908.00 J/kg. K, and 814.95 to 904.60 kg/m3, respectively. Among all pretreatments, UE20 could prefer for significant decreases in the drying time and improve drying characteristics of quince at the industrial extent.

Quality and mass transport properties of sugar beet roots under short duration, high airflow post-harvest storage

Active ventilation of stored sugar beet roots is used to control system temperature and slow the loss of quality that occurs with plant respiration and the growth of pathogens. Ventilation can at the same time increase rates of dehydration and dehydration related stresses of the stored sugar beet roots. Recent research into the use of forced ventilation to modify temperature in in-field sugar beet root stores has suggested that a controlled dehydration of roots could lead to improvements in quality. This work aimed to investigate the impacts to sugar beet root quality from short-term, high volume airflow during post-harvest storage. A modified environment experimental setup was developed. The experiment tested four airflow rates over three levels of ventilation duration, with the longest duration being seven days. Air temperature and relative humidity was constant within each seven day run, but varied between runs. Results showed that a mean weight loss of 11.7% with a corresponding increase in sucrose content of 13.2% was achieved at the highest airflow rate and ventilation duration. These changes in quality were a result of the transfer of water from the sugar beet roots. No reduction in total sucrose or other quality parameters was observed. The dependence of the rate of water transfer from the roots on airflow rate was quantified as the convective mass transfer coefficient. Estimates are given and cross-validated using dimensional analysis.

Development of infrared-assisted hybrid solar dryer for drying pineapple slices: Investigation of drying characteristics, mass transfer parameters, and quality attributes

The research aims to the development and performance evaluation of an infrared-assisted hybrid solar dryer (IR-HSD) for drying pineapple slices. The IR-HSD contains an evacuated tube solar collector, blower assembly, drying chamber, infrared heater, phase change material (PCM) chamber, etc. The study investigates and compares the thermal profiling, drying kinetics, mass transfer parameters and quality attributes of pineapple slices dried using different methods, such as direct drying, phase change material (PCM)-assisted drying, PCM + IR-assisted drying, and sun drying. The average temperatures achieved inside the drying chamber for direct drying and PCM-assisted drying were 60.16 °C and 57.29 °C, respectively, while the temperature reached 60 °C for PCM + IR assisted drying. The average drying rate in PCM + IR drying was higher than in direct and PCM-assisted drying methods. The best-fitting models were the Modified Page model for direct and PCM + IR-assisted drying and the Midilli-Kucuk model for PCM-assisted and sun drying. The higher effective moisture diffusivity for PCM + IR-assisted drying was 2.59 × 10−9 m2/s, followed by direct and PCM-assisted drying. Similarly, the convective mass transfer coefficient obtained was 1.085 × 10−7 m/s, 8.321 × 10−8 m/s, and 1.381 × 10−7 m/s for direct, PCM-assisted, and PCM + IR-assisted drying, respectively. The superior quality attributes such as total flavonoid content (TFC), total phenolic content (TPC), and antioxidant activity were observed in all drying approaches with retention of colour except open sun drying, which decreased significantly. FTIR spectroscopy analysis confirmed the presence of different bioactive, including aromatic compounds, phenols and hydrocarbon functional groups. The infrared-assisted hybrid solar drying of pineapple slices showed a better drying process and quality attributes.

Unified formulation of convective heat flux and mass flux vectors with thermodynamic coupling and related transfer coefficients

Based on the inherent thermodynamic relations of compressible species in binary mixtures at constant pressure, the general three-dimensional (3D) convective heat flux and mass flux vectors are formulated including the contributions of simultaneous advection, thermal diffusion and mass diffusion and their coupling effects between temperature and concentration gradients. The novel expressions for the diffusion-thermo and thermo-diffusion effects do not require the meager data of thermal diffusion ratio and are valid for both gas and liquid mixtures in laminar flows. The unified flux forms of the Maxwell's electromagnetic equation type are obtained in convective processes. It is further shown that the surface convective heat and mass transfer coefficients can be expressed in terms of the convective heat fluxes and mass fluxes away from surfaces in steady laminar flows, and that the expression of each transfer coefficient yields the identical form on both impermeable and permeable surfaces.

Numerical investigation of a novel tubular dew-point evaporative cooler

Dew-point evaporative cooling can efficiently bring down the air temperature to near dew point through water evaporation. Most dew-point evaporative coolers employing conventional plate type configurations have approached their limit and leveled off. To further improve the performance of dew-point evaporative cooling, a novel counter-flow tubular architecture is proposed in this paper. Based on the momentum, energy and mass balances, a rotating axisymmetric mathematical model is established for the new cooler. The heat and mass transfer process in the cooler is analyzed and compared with that of a conventional plate-type cooler. The cooling intensity, evaporation intensity and convective heat and mass transfer coefficient are discussed to elucidate the advanced cooling behavior of a tubular cooler. The results show that: (1) for ambient air with 30.0–38.0 ℃ temperature and 12.0–20.0 g/kg humidity, the product air temperature of a tube-type dew-point evaporative cooler is 1.6–3.0 ℃ lower than that of a plate-type, and the dew-point effectiveness is 0.18 higher; (2) the working air of the tubular cooler reaches humidity saturation at 0.05 m after entering the wet channel, which is significantly shorter than the plate structure by 0.25 m; (3) in the tubular wet channel, the channel length that achieves active cooling is longer than that of a plate-type cooler, and its convective heat and mass transfer coefficient are above 150 W/(m2·K) and 0.04 m/s respectively, i.e., 30 W/(m2·K) higher and 2.7 times to the plate-type cooler.

Study on the drying characteristics of green pellets of ultrafine iron ore concentrate

Ultrafine iron ore concentrate pose challenges like poor pellets formation performance, low-bursting temperature, and a complex drying thermal regime. To examine the drying characteristics of green pellets of ultrafine iron ore concentrate, we employed Weibull distribution function and Dincer model to fit and analyze the corresponding drying curve. The effects of drying temperature and air speed on the strength of dried pellets were also studied. Our findings reveal that the drying process of green pellets of ultrafine iron ore concentrate involves three stages: ascending speed, constant speed, and descending speed. As the drying temperature and air speed increase, the drying time decreases. The coefficient of determination R2 for the fitted Weibull distribution function model ranged from 0.995 to 0.998, while the R2 value for the Dincer model ranged from 0.990 to 0.996. Both fitted models align with experimental data and prove to be effective. According to the Bi values obtained through the Dincer model, raising the drying air speed in the initial stage and the drying air temperature in the subsequent stage of the drying system can efficiently remove moisture, reduce green pellets rupture risk, and maintain productivity. The moisture diffusion coefficient and convective mass transfer coefficient increase with rising temperature and air velocity, following the order of Deff > Dcal > D*eff as determined by the Weibull distribution function, Dincer model, and Fick's second law. Additionally, the activation energy value of ultrafine iron ore concentrate for drying derived from the Arrhenius formula is 4515.60 J/(mol?K). Notably, increasing drying temperature increases the strength of dried particles due to their more compact and dense internal structure. This study offers theoretical support for simulating the drying of green ultrafine iron ore concentrate pellets and provides guidelines for selecting diverse drying conditions and designing drying equipment.

Microstructural changes of young cement paste due to moisture transfer at low air pressures

It has been long recognized that low air pressure (AP) affects the moisture transfer in concrete, and thus its microstructure and bulk property, but less is known about the thermodynamic mechanism. This research investigated the microstructure evolution of young white Portland cement paste during wetting or drying at different APs (i.e. 101 kPa, 60 kPa, 20 kPa), the experimental and numerical analyses reveal the moisture transfer changes at low APs, which matters for the microstructural features of hardened cement paste. It was found that low APs increase the water vapor diffusion and convective mass transfer coefficients, and the intrinsic permeability of cement paste sample, which results in higher water contents within cement paste during wetting at 98 % RH but more rapid water losses during drying at 43 % RH. Owing to the distinct moisture transfer behaviors, the C-S-H structures were linked with water contents within confined pores.

Analysis of Mass And Heat Transfer Coefficients, Energy Consumption and Efficiency of in Dehydration of Cornelian Cherry

Drying is the process of removing water or other liquids from materials. However, drying is commonly used to remove water from food by thermal methods. Drying technology is a combination of transport phenomena and material science. Knowing the mass and heat transfer properties and the energy for the drying process is very important in terms of controlling and improving the process. In the present study, the effects of experimental parameters such as the temperature and velocity of air on mass and heat transfer coefficients, consumption and efficiency of energy in the dehydration of cornelian cherry were investigated. The cornelian cherry fruit was dried in a single layer in a dryer as convective at three different values (50, 60 and 70°C) of air temperatures and three different values (0.4, 0.7 and 1.0 m/s) of air velocities. The effective diffusivity values of moisture (Deff), coefficient of mass transfer (k), and Bi number were calculated using Dincer and Dost analytical approach. The heat capacity, thermal diffusivity and thermal conductivity are calculated as a function of moisture content. The convective heat and mass transfer coefficients were determined using engineering correlations and the Chilton-Colburn analogy. The consumption and efficiency of energy, thermal efficiency, and the efficiency of drying were analyzed for all experimental conditions.

Modelling the drying characteristics of human faeces using thin-layer drying models and calculation of mass transfer properties at ambient conditions

Convective drying was used to investigate the drying characteristics of human faeces. Thin-layer drying semi-empirical models were used to describe the drying characteristics of human faeces under varying conditions of temperature, air velocity and relative humidity. The drying experiments were conducted at moderate near-ambient conditions, with temperatures of 20 °C, 25 °C, 30 °C, 35 °C and 40 °C, and air flows of 0.1 m/s, 0.2 m/s and 0.5 m/s, at relative humidity of 5%, 10%, 20% and 40%, and initial moisture content of 82.2%, 78.1%, 71.7%, 71.1% and 64.2%. The experimental data curves for faeces for varying temperatures were fitted to the Newton, the Henderson and Pabis, the Page, and the Midilli-Kucuk models, while other experimental conditions were kept constant. Page model was the best fitting model, with R2 values ranging from 0.9950 to 09,977. Effective moisture diffusivity and convective mass transfer coefficient were found to vary from 8.81 × 10−8 m2/s and 1.31 × 10−5 m/s at 20 °C to 9.57 × 10−7 m2/s and 3.49 × 10−4 m/s at 40 °C, respectively. The activation energy was found to be 42.38 kJ/mol. The results presented in this study confirm that human faeces can be dried under moderate ambient conditions by using solar and wind energy. The modelled kinetics can inform the development and design of a dry sanitation system that does not require electricity or any other fuels for heating and air convection.

Heat and Mass Transfer in Structural Ceramic Blocks: An Analytical and Phenomenological Approach

The ceramic industry is one of the pillars of the Brazilian economy, characterized by making low-cost products and an obsolete manufacturing process from a technological point of view. Among the various stages of production of ceramic materials, drying is one of the most energy-consuming and, in general, causes structural damage to the product, compromising its mechanical performance and final quality. Despite the relevance, studies on the drying of ceramic materials are mostly conducted at the experimental level and limited to some specific operational conditions. In this scenario, this research aims to theoretically study the heat and mass transfers in industrial ceramic blocks during drying. Based on the lumped analysis method, and considering the dimensional variations of the material, new phenomenological mathematical models and their respective analytical solutions are proposed to describe the kinetics of mass loss and heating of the material. The predicted results referring to the thermal and gravimetric behavior of the block during the oven drying process under different conditions are compared with the experimental data, obtaining excellent agreement between the results. Furthermore, the transport coefficients were estimated, proving the dependence of these parameters on the drying air conditions. The convective mass transfer coefficient ranged from 6.69 × 10–7 to 15.97 × 10–7 m/s on the outer surface of the block and from 0.70 × 10–7 to 1.03 × 10–7 m/s on the inner surface of the material when the drying air temperature ranged from 50 to 100 °C. The convective heat transfer coefficient ranged from 4.79 to 2.04 W/(m2.°C) on the outer surface of the block and from 1.00 to 0.94 W/(m2.°C) on the inner surface of the material when air temperature ranged from 50 to 100 °C.

Drying and rehydration kinetics of peeled and unpeeled green apple slices (Granny Smith cv)

In this work, the kinetics of drying and rehydration of green apple slices peeled and unpeeled (Granny Smith cv) were studied. The apple slices were dried at 50, 60, and 70 °C, and after that, rehydrated at ambient (Ta ) and boiling temperature (Tb ). The drying kinetics were adjusted with the Dincer and Dost model, giving a good fit. Effective diffusivity (Deff ) and the convective mass transfer coefficient (hm ) were also determined, both coefficients increase with drying temperature, being 1.25 × 10−9 m2 s−1 and 9.53 × 10−7 m2 s−1 the highest values obtained for the peeled apple slices respectively. Peleg and Weibull models were adjusted to the rehydration experimental data obtaining a good fit (R 2 > 0.99). Deff values increase significantly with rehydration temperature but take similar values between peeled and unpeeled samples. Acidity, pH, moisture content, solid soluble content, and equivalent diameter were determined to compare the fresh apple slices with those after dehydration and the post-rehydration process. The apple slices rehydrated at boiling temperature better preserved the characteristics of fresh samples due to the short immersion times in water, no significant differences were observed between peeled and unpeeled apples. According to the results, it is convenient to dry the apple slices unpeeled at 70 °C and rehydrate them at Tb .

Convective and evaporative heat transfer coefficients during drying of ivy gourd under natural and forced convection solar dryer.

In the present work, a study on convective heat, mass transfer coefficients and evaporative heat transfer coefficient of the thin layer drying process of ivy gourd is performed. The experiment was conducted in three drying modes such as natural, forced convection solar dryer and open sun drying. The hourly data for the rate of moisture removal, sample temperature, relative humidity inside and outside the solar and ambient air temperature for complete drying have been recorded. The drying air temperature varied from 55, 65, 70 and 75°C, and the air velocity was 1, 1.5 and 2m/s. All the drying experiments had shown a falling rate period. The data obtained from experimentation have been used to evaluate the experimental constant values of C and n by simple regression analysis. Based on the values of "C" and "n", convective and evaporative heat transfer coefficients for ivy gourd were determined. The average convective heat and mass transfer coefficients varied between 2.64 and 8.30W/m2 °C and 0.0025 to 0.0076m/s for temperature ranges, at the different air velocities, respectively. The average evaporative heat transfer coefficient for ivy gourd varied from 181.89 to 421.84W/m2 °C. It was observed that convective and evaporative heat transfer coefficients increase with the increase in drying air temperature. The rate of increment of evaporative heat transfer coefficient is higher than the convective heat transfer coefficient. The intensity of heat and mass transfer during solar drying depends on the drying air temperature and velocity.

Drying quality and energy consumption efficient improvements in hot air drying of papaya slices by step-down relative humidity based on heat and mass transfer characteristics and 3D simulation

Convective hot air drying technology is widely used for fruits and vegetables drying process due to its simplicity in usage, small investment, and convenience in operation. However, it is generally considered energy-intensive process with lower drying efficiency and quality because of unprecise control of drying process. In order to solve these problems, step-down relative humidity (RH) hot air drying technology was applied for papaya drying to improve drying efficiency and quality and reduce energy consumption. The effect of constant RH (20%, 30%, 40%, and 50%) and step-down RH of 50% with different holding times (10, 30, 60, and 90 min) and then decrease to 20% on heat and mass transfer process, drying quality, and energy consumption was examined. Results showed that decreasing RH contributed to improvement in the drying rate (DR). Compared to constant RH of 20%, the drying time under initial RH of 50% for 30 min followed by 20% RH shortened by 21.4%. Additionally, under this RH control strategy, the rehydration ratio (RR) and energy consumption achieved their maximum (7.71 ± 0.06) and minimum value (0.015 ± 0.001 kJ/g), respectively. During initial drying stage, high RH improved convective heat transfer coefficient (ht ) values and material temperature increased quickly. Furthermore, high RH contributed to maintaining pore structure formation, which benefited moisture diffusion. When RH was reduced, low RH enhanced convective mass transfer coefficient (hm ) significantly, resulting in rapid diffusion of internal moisture at high temperature, as well as rapid moisture evaporation from the surface. Therefore, step-down RH hot air drying was verified to improve drying of papaya slices, making it energy efficient process. The findings in current work indicate step-down relative humidity strategy is a promising technique to enhance hot air drying process, quality, and energy efficiency of papaya slices.