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In this paper, time changes of the Brownian motions on generalized Sierpinski carpets including

The volume-of-fluid (VOF) method, supplemented by models of interfacial heat and mass transfer, is a universal and very effective tool for simulation and detailed analysis of intricate processes occurring in systems with phase transitions. The key feature of this method is that it can quite accurately and in detail describe the physical pattern of running processes in the presence of a sharp phase boundary and provide quantitative data on the distribution of local heat-transfer characteristics and the dynamics of the interphase boundary and associated phenomena, thereby making the VOF method advantageous in researches and engineering practice. Development and improvement of heat and mass transfer models and efficient numerical VOF algorithms, as well as preparation of recommendations for the application of these approaches, are an urgent problem. This paper proposes an approach to the prediction of interfacial heat and mass transfer rate, which is based on the analysis of phase transitions in single-component systems using the linear theory of nonequilibrium processes. The results are presented of verification calculations performed for several standard problems. The classical problems of one-dimensional boiling and condensation (the Stefan problem) are examined as are such problems as vapor condensation in tubes of different orientations, condensation from stagnant or moving vapor on the surface of smooth horizontal tubes, and film boiling on the surface of horizontal cylinders. The predictions are verified against classical solutions and available experimental data. Calculations were carried out for fluids with different thermophysical properties, including water, pentane, propane, R-113, R-21, and R-142b. The maximum ratio of the densities of liquid and vapor phases was as high as 1600 (water at atmospheric pressure). The simulation results demonstrate the versatility of the proposed approach, which allows us to recommend it for solving a variety of engineering problems.

Tomato ( Solanum lycopersicum ) and brinjal ( Solanum melongena ) are food products that are frequently wasted in Indian markets as their shelf lives are only a few days. An indirect type solar dryer (ITSD) has been developed. It’s performance and drying kinetics of brinjal and tomato slices have been analyzed. The moisture content of tomato decreased from 15.667 to 0.803 kg/kg of dry basis (db) and that of brinjal reduced from 10.111 to 0.498 kg/kg of db. The drying curve was fitted with the different models of existing studies. The average effective moisture diffusivity is estimated and it is 3.60 × 10 −9 and 4.00× 10 −9  m 2 /s for tomato and brinjal, respectively. Mass transfer coefficient was in the range of 0.82 × 10 −4 to 2.85 × 10 −3  m/s for tomato and 1.11 × 10 −4 to 3.32 × 10 −3  m/s for brinjal. The heat transfer coefficient was in the range of 0.089 to 2.888 W/m 2  K and 0.1066 to 3.3564 W/m 2  K for tomato and brinjal, respectively. The activation energy for tomato and brinjal was 21.19 and 19.46 kJ/mol, respectively. The average thermal efficiency of the collector and dryer was 59.05% and 31.4% during tomato drying and 58.42% and 25.16% for brinjal drying, respectively.

The Guyer–Krumhansl equation for the heat flux is a phenomenological bridge between Fourier heat transport (for size of the system much bigger than the mean free path of heat carriers) and hydrodynamic heat transport (for size of the system comparable to the mean free path of heat carriers). The corresponding phonon hydrodynamics is analogous to Newtonian hydrodynamics, but with the velocity replaced by the heat flux, the pressure gradient replaced by the temperature gradient and the shear viscosity replaced by the square of the mean-free path divided by the thermal conductivity. In this paper, we propose a nonlinear generalization of the Guyer–Krumhansl equation and phonon hydrodynamics based on an analogy with the power-law model of non-Newtonian fluids leading to a non-diffusive behaviour of heat transport. On the basis of this model, we obtain the corresponding nonlinear effective thermal conductivity of the model, depending on the radius of the channel and on the temperature gradient. The present proposal could be useful in the light of recent analyses of Poiseuille phonon hydrodynamics which suggest a non-Newtonian behaviour.

Microchannel heat sinks (MCHS) belong to one of the most prominent methods of passive cooling of microelectronics. In this work, a circular microchannel-based MCHS was installed over a microelectronic mimicking heated surface, which was subjected to 50 to 125 kW/m 2 , and the convective cooling of MCHS was studied using nanofluids of copper (Cu) and carbon nanotube (CNT) [both at 0.05 wt % concentration in de-ionized (DI) water] as coolant, along with DI water. The experimental results suggest that the nanofluid-cooled MCHS, especially the CNT one, outperformed the pure water-cooled system, with significantly higher heat transfer coefficient (HTC), and lower pumping power, rendering the former system more energetically favorable. At a flow rate of 60 ml/min and heat flux of 100 kW/m 2 , the HTC enhancements in water + CNT and water + Cu were 15.7 and 6.2% more than water, respectively. Due to addition of surfactant in DI water for suspending CNT, an apparent slip flow became prevalent in the microchannel, leading to a significant pressure drop reduction while pumping water + CNT. This observation helped in gauging the total power saving that can be accessed using water + CNT, if one follows periodic heating/cooling between an upper critical temperature and safe temperature range rather than continuous cooling of the electronic surface.

The results of the investigation into heat-transfer enhancement at increasing critical heat flux due to modification of a wall’s inner surface are presented. The greater need for new, compact, and energy-efficient heat exchangers on the basis of minichannels for high-tech industries makes this investigation urgent. The potential for application of small diameter channels in systems where various dielectric liquids or freons at moderate and high reduced pressures can be used as a coolant is being actively investigated today. The experiments were performed in a heated vertical minichannel. The wall was modified by the rolling method, which has not yet been used in small diameter channels. The experiments were performed with a forced flow of R125 refrigerant at high reduced pressures of 0.43 and 0.56 in the range of mass flowrates from 200 to 1200 kg/(m 2 s), which is the most applicable range for minichannel heat exchangers. Heat transfer during forced convection and flow boiling was studied. The experimental setup and the minichannel inner wall modification method are described. Experimental data on forced convection and flow boiling heat-transfer coefficients, critical heat fluxes, and pressure drops are presented. The heat-transfer data were compared with the results obtained previously with the inner surface modified by the action of laser pulses on the outer wall. The convective heat-transfer coefficient in a minichannel with the inner surface modified by rolling was found to be much greater than that in a smooth channel. The obtained convective heat-transfer coefficients are compared with the predictions by empirical formulas derived for large-diameter pipes with the wall surface modified by rolling.

The Volume of Fluid (VOF) method supplemented with heat and mass transfer models at the interphase boundary is actively employed in the investigation of film condensation and film boiling, in the calculation of evaporators, for predicting the dynamics of vapor bubble collapse in a pool of subcooled liquid, or for other purposes. The original VOF algorithm proposed by Hirt is intended for the simulation of a single-phase incompressible liquid with a free boundary at which a constant pressure is specified. The extension of the VOF-algorithm to a two-phase fluid, especially with mass transfer, is not a common problem from the standpoint of the rigor of mathematical formulation. In our previous studies, approaches have been developed to the 2D and 3D simulation of heat and mass transfer processes during vapor condensation on the surface of horizontal smooth tubes, and condensation on a smooth tube bundle was simulated in 2D formulation. This paper presents the results of 3D simulation of R-21 refrigerant condensation in a small-sized tube bundle. Characteristics of the tube bundle are the same as those of the tube bundle tested at the Institute of Thermophysics of the Siberian Branch of the Russian Academy of Sciences (SB RAS) (tube diameter is 16 mm, transverse pitch is 26 mm, longitudinal pitch is 15 mm). The condensation was examined in saturated vapor flow at a temperature of = 333.15 K incoming onto the tube bundle at a velocity of up to 0.9 m/s. The 3D predictions agree qualitatively and quantitatively with the 2D predictions and the experimental data. The distribution of condensate in the tube bundle is presented. The spectrums of fluctuations in the average heat transfer for tubes are analyzed. It is pointed out that the thermal boundary layer development region induced by the condensate falling from the upper to lower tubes should be considered.

The purpose of the review is to find the best currently available correlations for calculating heat transfer and pressure drop in the main heat-transfer equipment items in organic Rankine cycle (ORC) units. The search is limited to the designs of apparatuses, which are the best ones in the opinion of the authors of this paper, for a conventional two-circuit ORC-unit, where thermal oil cools a heat source in the first circuit and transfers heat to refrigerant in the vapor generator (hereinafter referred to as the evaporator). Besides the evaporator, the second circuit of the unit includes a “refrigerant–water” or “refrigerant–air” condenser and a regenerative heat exchanger which heats up liquid refrigerant upstream of the evaporator with the exhaust vapor of the turbine (or expander). The criteria are presented for selecting working fluids for such units depending on the heat source temperature. The working fluids that have found the widest application at each temperature level (such as cyclopentane, benzene, toluene, MM, MDM, R1233zd, R245fa, R601, R601a, RC318, R134a) are listed, and their characteristics and thermodynamic properties are presented at specified condensation (25°C) and boiling (200, 120, and 70°C) points. The analysis of these data, including information on the proposed working fluids, has yielded nominal parameters of ORC-units. Thousands of fundamental and engineering works are devoted to the study of boiling and condensation processes, the interest in which has been growing over the past 10–15 years. The development of new energy conversion technologies and the appearance of new working fluids, materials, and methods of surface treatment has given a second wind. This paper reviews correlations for heat-transfer coefficients and hydraulic resistance factors in apparatuses with refrigerant boiling in round tubes, condensation in tubes and channels and in the shell side (on tube bundles), and heating and cooling of single-phase refrigerant in tubes and channels. The correlations for engineering calculation of the main heat-transfer equipment of ORC-units, which are the most convenient ones in the authors’ opinion, are presented.

Results are presented for theoretical and experimental investigations into the kinetics of dehydration and heat and mass transfer in a vibrofluidized bed of vegetable origin materials using the example of potato and carrot particles in convective and radiative-convective heat inputs. A comparison has been drawn between calculation results and experimental data. The contribution of infrared radiation and convective heat flux has been determined. The possibility and regimes of dehydration process enhancement and reduction of its duration in a vibrofluidized bed in radiative-convective heat input have been shown.