A mathematical model for a thermally coupled humidification–dehumidification desalination process

Pressure effect on an ocean-based humidification-dehumidification desalination process

This paper presents experimental results on the study of the effects of ejector adiabatic absorber on heat and mass transfer of binary nanofluid with heat transfer additives (2-ethyl-1-hexanol and gum Arabic). In this case, H2O/lithium bromide-alumina nanofluid was suggested due to a growing interest in absorption heat transfer working fluid for solar energy application. An experimental setup — ejector test rig — was designed to study the absorption, heat, and mass transfer rate as a result of refrigerant vapour mass flow entrained by the ejector adiabatic absorber. The study was carried out at different solution mass flowrate (0.051 to 0.17 kg/s) with three prepared sample solutions, which include pure LiBr solution, LiBr-Alumina nanofluid without heat transfer additives, and LiBr-Alumina nanofluid with heat transfer additives. The absorption rate, mass transfer coefficient, heat transfer rate, and heat transfer coefficient for the three samples were reported. On the other hand, the percentage enhancements for all the parameters — at a suitable flow rate of 0.085 kg/s — due to the addition of alumina without and with heat transfer additives were recorded. The absorption rate enhancements were 25% and 96%, the enhancement rates of mass transfer coefficient recorded were 20% and 82%, the heat transfer rate enhancements were 85% and 183%, and the heat transfer coefficient enhancements obtained were 72% and 156% with addition of alumina nanoparticles only and alumina nanoparticles with heat transfer additives respectively. Material mass balance analysis suggests that mass inflow in the ejector equals to the mass outflow from the ejector, indicating a complete absorption of the entrained refrigerant vapour beyond which falling film absorption can occur due to concentration. This article also presents experimental evidence of the capability of ejector as strong adiabatic absorber, heat, and mass transfer component, which were earlier reported using numerical models.

Solar desalination based on humidification process—I. Evaluating the heat and mass transfer coefficients

Effect of humidifier characteristics on performance of a small-scale humidification-dehumidification desalination system

Experimental mass and heat transfer at a serpentine tube heat exchanger located at the wall of a square stirred tank reactor

Solid-state fermentation (SSF) involves the growth of microorganisms on moist solids in the absence of free water. Unlike submerged liquid fermentation (SLF), SSF is not used extensively at large scales, despite potential advantages of SSF over SLF for the production of some biotechnology products. This is because quantitative rules for the design, operation and scale-up of SSF are currently lacking. The development of such rules has been hampered by difficulties in studying SSF systems and by the poor understanding of the effects of bioreactor design and operation on SSF performance. Several bioreactor types have been used for SSF at small scales. One that has potential at large scales is the rotating drum bioreactor (ROB). This thesis investigates the operation of small-scale ROBs for SSF with the filamentous fungus, Aspergillus oryzae. It aims to improve understanding of the effects of operational variables on SSF performance, and thereby provide a basis for further research into the design and operation of ROBs for SSF at large scales. The work presented in this thesis consisted of several steps: 1. Development of experimental systems. Two identical, glass 18.7 l RDBs were constructed with length to diameter ratios (L/D) of 4.4. The ends were fined with metal caps which permitted gas flow from one end of the bioreactor to the other. The bioreactors were able to be rotated at a wide range of speeds in an enclosed unit. The air temperature in the unit was controlled at 32°C during all fermentations. Two substrate types were used during SSFs conducted in the 18.7 l RDBs: an artificial gel based substrate and steamed wheat bran. The gel substrate consisted of 6 mm cubical particles, had a starch concentration of 4.4 % (w/w) and contained only inorganic nitrogen. Biomass of A. orvzae during gel SSFs was estimated by protein. Growth of A. oryzae during wheat bran fermentations was monitored by oxygen uptake. 2. Experimental investigations of the effects of operational variables on growth of A. oryzae during SSFs in small-scale RDBs. Gel substrate fermentations were conducted using 1.5 or 3.0 kg quantities in the 18.7 l RDBs. Under stationary conditions, the fungus grew well and evenly throughout the substrate bed. A. oryzae also grew well during all rolled fermentations, although shear forces caused by drum rotation reduced overall protein production and sporulation. The highest recorded substrate temperature was 35.5°C which occurred during a stationary 3 kg SSF. The highest oxygen uptake rate (OUR) recorded during a gel fermentation was 38 µmole. (g gel substrate)-1.h-1. Fermentations of 2 kg wheat bran were also conducted in the 18.7 l RDBs. The fungus grew poorly and unevenly during stationary fermentations. Drum rotation improved overall growth of A. oryzae compared to static fermentations. Rotational speed had no apparent effect on fungal growth during rolled fermentations. Any adverse effects of shear caused by agitation of the substrate, were outweighed by improvements in mass and heat transfer. Peak OURs in excess of 250 µmole.(g wheat bran)-1.h-1 were recorded during rolled fermentations. In all fermentations, maximum substrate temperatures were greater than 40°C. The fungus also grew well during rolled, 10 kg wheat bran SSFs in a 200 l RDB system. During rolled SSFs in the 18.7 l RDBs, the substrate appeared homogeneous in the radial direction (ie. within the substrate depth) but not in the axial direction (ie. along the bioreactor length). Substrate near the aeration inlet end appeared drier and was cooler than substrate near the gas exit end of the bioreactors. 3. Experimental investigations of the effects of operational variables on gas flow behaviour in small-scale RDBs. Gas flow behaviour in the 18.7 l RDBs was investigated under similar conditions to those used in wheat bran fermentations. Gas was fed continuously into the bioreactors during residence time distribution experiments. Air was replaced by pure nitrogen for a 5 min period and the oxygen depletion response was measured in the gas stream leaving the bioreactor. The response curves for various operating conditions showed that high rotational speeds and low gas flow rates promoted axial dispersion of the gas molecules as they passed through the bioreactors. Overall, the experiment studies (steps 2 and 3) showed that: • Optimal aeration characteristics are more likely to be determined by metabolic heat removal than by oxygen supply requirements. • Shear forces may adversely affect fungal growth, but their effect is not critical since the fungus may still grow well under high shear conditions. • Substrate characteristics, such as particle size and nutrient concentrations are important for SSFs conducted in RDBs since they affect the total metabolic activity in the bioreactor and therefore, operating requirements. • Substrate homogeneity in the radial direction may be achieved by agitation of the substrate during drum rotation. However, substrate homogeneity in the axial direction is difficult to achieve in RDB designs with end-to-end aeration. 4. Proposal of design and operation features of RDBs for large-scale SSF. Heat removal, oxygen supply, and homogeneous conditions in the substrate (in both the radial and axial directions) will be important during fungal pure-culture, batch SSFs conducted in RDBs at large scales. Baffles and large L/Ds could be useful for such operations since these will enhance interactions between the solid and gas phases in the bioreactor and thereby improve the rates of heat and mass transfer. The design of aeration systems for large-scale RDBs should attempt to achieve gas phase behaviour analogous to a number of ideal stirred tanks in parallel. This will result in homogeneous conditions in the bioreactor headspace in the axial direction. Since the rates of heat transfer and mass transfer between the substrate and the headspace will then be equal along the bioreactor length, substrate homogeneity in the axial direction should also be achieved. 5. Development of rule-of-thumb scale-up criteria for SSF in RDBs. Simple rule-of-thumb scale-up criteria, including constant L/D, constant solids loading, constant aeration rate (in vvm) and constant fraction of the drum critical speed of rotation, were explored. However, scale-up based on these criteria would lead to overheating problems. 6. Proposal of a mathematical model which mechanistically describes heat transfer during SSF in RDBs. A mathematical model was proposed which mechanistically describes heat transfer during SSF of A. oryzae in a RDB. The model incorporates several mechanisms for heat transfer between the substrate, bioreactor headspace, bioreactor wall and the surroundings. These mechanisms include conduction, convection, evaporative cooling and energy transfer related to gas entering and leaving the RDB system. The model was used to predict SSF performance at a number of scales and under various design and operating conditions. The small scale predictions compared well with experimental results. The predictions of large scale systems showed that aeration requirements (in vvm) increased significantly with scale. These predictions also showed that water loss from the substrate was the dominant mechanism of heat removal at large scales. Although not validated, the model was shown to be a useful tool in exploring the design and operation of RDBs for SSF. It represents a first step towards developing semi-fundamental scale-up criteria for SSF in RDBs. Overall, the experimental studies conducted as part of this research have contributed significantly to the understanding of the operation of RDBs for SSF. However, further work and different experimental approaches are needed for continued research in this area. The holistic approach (as used in step 2) will be useful for characterising the overall effects of bioreactor design and operation on SSF performance. However such studies will need to be complemented by other experiments where transport and microbial phenomena are characterised independently of each other (such as in step 3). This should lead to better understanding of SSF performance in RDBs and permit the development of accurate mathematical models incorporating important microbial and transport phenomena. Mathematical models based on mechanistic descriptions of processes occurring during SSF in RDBs will facilitate the development of useful scale-up strategies.

In the current research, the effect of thermophoretic motion combined with temperature-dependent thermal conductivity on natural convection flow around the surface of a sphere at several circumferential locations has been presented. The modeled nonlinear governing partial differential has been transformed into a dimensionless form with the help of appropriate non-dimensional variables. Later, the finite difference method is applied to solve the proposed model. The effect of controlling parameters, such as thermal conductivity variation parameter γ, Prandtl number Pr, Schmidt number Sc, thermophoretic coefficient k, and thermophoresis parameter Nt on the velocity field, temperature distribution, mass concentration, skin friction, rate of heat transfer, and rate of mass transfer has been highlighted. The estimations of the emerging parameters on the physical properties are displayed in graphical and in tabular forms. It has been predicted that the rise in γ, Nt, Sc, Pr, and k increases the velocity distribution, but the reverse behavior has been seen in the temperature field. The enhancement in Nt, Sc, Pr, and k boosts up the curves of mass concentration, and the rise in γ suppresses the concentration function. It has been observed that an increase in γ reduces the skin friction and the rate of mass transfer but opposite behavior of the rate of heat transfer occurs. Furthermore, increasing values of Sc cause the skin friction to lose the dominance in the rate of heat and mass transfer. It has been also noticed that increasing values of Nt strengthen the skin friction and rate of heat transfer, and attenuation occurs in the case of the rate of mass transfer.

The mixed Marangoni assisting/resisting flow of a nanofluid with thermal radiative heat flux is analyzed when thermal and solutal buoyant forces are significant. The heat and mass transfer rates are simultaneously optimized by utilizing the Response Surface Methodology (RSM). The face‐centered Central Composite Design (fc‐CCD) is used for the numerical experimental design involved in RSM. The sensitivities of the heat and mass transfer rates are evaluated to compare the impact of the thermal and solutal buoyant forces. Appropriate scaling and similarity transformations are utilized to simplify the problem and then numerical solutions are obtained. The nanoliquid flow, temperature, and concentration profiles are plotted for the buoyancy assisting and opposing Marangoni cases. The Marangoni flow with opposite buoyancy is found to have a greater magnitude of velocity while the flows assisted by the buoyancy have a greater magnitude of temperature and concentration profiles. Thermal buoyancy force has a predominant (0.6%) impact on both heat and mass transfer rates compared to solutal buoyancy force. Buoyancy forces are positively sensitive to heat and mass transfer rates. The thermal radiation aspect augments the temperature profile throughout the domain. The optimized mass and heat transfer rates ( and ) is achieved at the highest level of the buoyancy forces and ratio of Marangoni numbers.

Rotor-stator spinning disc reactor

Practical applicationsAnalysis of Casson nanofluid transport rates near a vertical stretching sheet with dissipation and slip effects will provide relevant information for practitioners to make informed decisions in handling real flow systems. Hence, the present study will contribute in not only supplementing the theoretical gaps for the scientific community but also improving the working efficiency of practical flow systems in manufacturing industries and the quality of their products.PurposeThis study mainly focused on examining the rates of hydromagnetic transport phenomena of Casson nanofluid near a vertical surface in the presence of slip, dissipation, and cross-diffusion effects. Based on the underlying conservation laws in physical sciences and significant model assumptions, a more comprehensive mathematical model is taken into account. Efforts are made to analyze variations in the rates of heat, mass, and momentum transfer against the continuous change of the variables.MethodologyThe solutions for the resulting model equations are explored with the help of the optimal homotopy analysis method.FindingsAmong the results of the study, it is determined that the rate of heat transfer between the solid surface and the surrounding fluid is enhanced by increasing the effect of magnetic field (B > 3.5), thermal radiation (Rd > 2.5), or concentration buoyancy force (Gc > 5). On the other hand, the mass transfer near the solid surface can be assisted by increasing the effect of thermal diffusion (Sr > 0), heat generation (Q > 2), thermal radiation (Rd > 2.5), and concentration buoyancy force (Gc > 3). Furthermore, the rate of momentum transfer of the fluid flow near the solid surface can be facilitated by increasing the effect of flow unsteadiness (A > 2.5) or heat sink (Q < −4).OriginalityMost of the available studies on the physical quantities of practical interest were made based on presenting their variations at only some selected values of the parameters. Such analysis cannot give full information about the complete behavior of the quantities in response to the governing parameters. Thus, in this study a considerable attention is given to how the fluid transport rates vary with the relevant factors in a continuous domain of the parameters. Furthermore, the study considers a more comprehensive mathematical model in the area under consideration and the resulting equations are solved by an efficient optimal homotopy analysis method.

The growing demand for drinking water has led to the exploration of sustainable and efficient alternatives in the desalination process. Conventional methods, such as reverse osmosis and flash multistage evaporation, face challenges in small-scale applicability, especially in rural areas with electricity access restrictions. This article focuses on comparing these technologies with the innovative air humidification-dehumidification (HDH) process for the production of 100 liters/hour of desalinated water. Mass and energy balances were used to evaluate the efficiency of each technology, specifically the amount of thermal energy needed per volume of desalinated water, and the area of solar collectors necessary for the process. Open and closed circuit configurations with and without regeneration were considered. Initial results reveal that in an open-loop configuration without regeneration, the thermal energy consumption in the HDH process is 3,6 and 5,5 times higher compared to the MSF and MED processes, espectively. In conclusion, despite higher thermal energy consumption, compared to MED and RO technologies, the HDH process presents significant advantages in terms of reduced initial and maintenance costs. Furthermore, the modularity of the system allows it to be adapted to various water demands, from small quantities to more significant volumes. In this way, the HDH process is an effective and accessible solution for water desalination in areas with limited access to electricity and maintenance. The combination of low costs and the use of renewable sources reinforce the potential for implementation of this technology in diverse environments, from rural areas to more extensive applications.

A boundary layer analysis has been presented for the natural convection flow of a non-Newtonian nanofluid past a sphere. Solutions of the set of nonsimilarity equations are obtained by employing the implicit finite difference method together with Keller box elimination method. Numerical results for friction factor, surface heat transfer rate and mass transfer rate have been presented for parametric variations of the material parameters, buoyancy ratio parameter [Formula: see text], Brownian motion parameter NB, thermophoresis parameter NT and Schmidt number Sc. The dependency of the surface heat transfer rate (Nusselt number) and mass transfer rate on these parameters has been discussed. It was found that the heat transfer rate decreases and mass transfer rates increase as Schmidt number increases. The friction factor and heat transfer rates decrease as the cross viscosity parameter [Formula: see text] increases. The heat transfer rates increase and mass transfer rates decrease as the buoyancy ratio parameter N increases. As the thermophoresis parameter NT increases, the heat and mass transfer rates increase. As the Brownian parameter NB increases, the heat and mass transfer rates decrease. Brownian motion decelerates the flow in the nanofluid boundary layer. Brownian diffusion promotes heat conduction. The Brownian motion and thermophoresis of nanoparticles increase the effective thermal conductivity of the nanofluid. Both Brownian diffusion and thermophoresis give rise to cross diffusion terms that are similar to the familiar Soret and Dufour cross diffusion terms that arise with a binary fluid.

Biogas fueled combined cooling, desalinated water and power generation systems

Rotating foam reactors: Mass transfer and reaction rate

This present examination researches the impacts of thermophoresis, heat source and Hall current on dissipative adjusted MHD joint convection stream about an inclined plate inserted in a permeable medium. Utilizing dimensionless variables, the system of partial differential equations is changed into dimensionless equations. By making use of perturbation technique, estimated solutions for velocity, temperature, concentration profiles, skin friction, rate of heat transfer and rate of mass transfer have been determined. The attained results are explained with an assistance of diagrams to examine the impact of distinct parameters such as Magnetic parameter (M), Aligned magnetic parameter (ξ), Schmidt number (Sc), Eckert number (Ec), inclined angle (α), Prandtl number (Pr), heat generation parameter (Q), and chemical reaction (Kr), assuming two cases viz. Case I: Gr < 0, Gm < 0 (flow on heated plate); Case II: when Gr > 0, Gm > 0(flow on cooled plate). Additionally, the impacts of the appropriate parameters on the skin-friction coefficient and rates of heat and mass transfer are numerically furnished in tabular form. Skin friction coefficients are firmly diminished as magnetic field rises. Sherwood and Nusselt numbers boost up as enhance in chemical reaction.