Cutting edge research and new technologies in heat and mass transfer processes of refrigeration and air conditioning systems

Currently, there is a problem of developing effective methods to calculate heat and mass transfer processes during steam condensation from a vapor-gas mixture in industrial devices. This is due to the need to create new reliable and highly efficient designs of heat exchangers for various purposes. The finite element method has been used in the ANSYS Fluent software package during the numerical simulation. An experimental plant is tested in the pipeline of an industrial enterprise at the production site of the Technopolis KHIMGRAD industrial park (Kazan-city). The applicability of the Lee model to solve problems of water flow in a pipeline with partial evaporation is proved. This model allows you to accurately account for the processes of evaporation and condensation, which is important for the design of cooling and air conditioning systems. The authors have designed a three-dimensional model of the flow area of the experimental module and a CFD model to calculate temperatures, phase velocities and their concentrations, considering the peculiarities of the ongoing processes of heat and mass transfer. A slight effect on the heat flow of a 180° rotation during the flow of a liquid medium in the range of the Reynolds numbers 1800–2600 is shown. The developed model is verified with the results obtained at the experimental plant. It is established that the model reproduces experimental data well. An analysis of studies of heat and mass transfer processes during the fluid flow in channels in the presence of phase transitions is carried out. The problems of calculating the parameters of heat transfer during the movement of a fluid in a transient mode are revealed, especially at values of the Reynolds number close to 2000. The experimental plant diagram is presented. It is found that in the range of Reynolds numbers of 2600–3600, the calculated temperature values differ from the temperature values obtained experimentally by less than 0,42 %. During the partial transition of water into steam in the range of Reynolds numbers of 1800–2600, the deviations were less than 6 %, which confirms the adequacy of numerical modeling of calculating the heat transfer process in a pipe. It is shown that considering the influence of temperature on surface tension, the coefficient of thermal conductivity of water and the coefficient of dynamic viscosity of water is important to obtain accurate results. The proposed approach can be used to optimize the operating parameters of condensers and evaporators, as well as to accurately calculate the heat transfer coefficient and determine the areas of vapor formation. It will improve the energy efficiency of industrial installations and reduce the cost of equipment operation.

Liquid desiccant system is more and more popular in the air-conditioning system. This paper focuses on the match property analyses of the internally-cooled liquid desiccant system. The unmatched coefficients both of the sensible heat transfer process between the desiccant and the cooling medium and the coupled heat and mass transfer process between the desiccant and the air were defined for the internally-cooled device. Numerical model of the dehumidifier was established for calculating the unmatched coefficients. Match properties of different kinds of liquid desiccant systems under various boundary conditions were then compared. The results show that the internally-cooled device has much smaller unmatched coefficients compared to the two-flow devices (adiabatic dehumidifier and sensible heat exchanger). The unmatched coefficients are good indicators for system improvement with identical heat and mass transfer area.

Challenges and opportunities in the management of refrigeration and air conditioning systems to reduce environmental impacts in the Colombian health sector

Acoustic streaming outside spherical particles and parameter analysis of heat transfer enhancement

Nano-enhanced refrigerant is one of the promising heat transfer fluids in refrigeration systems. It has the capability to boost the efficiency of vapour compression refrigeration and air conditioning systems. In heat transfer application, the density of the fluid plays a crucial role in identifying various heat transfer characteristics such as the Reynolds number, Nusselt number, the friction factor and the pressure loss. Compared to thermal conductivity and viscosity, determining the density of nanofluids has received very less attention in research. The liquid and vapour densities of alumina (Al2O3), titanium dioxide (TiO2) and cupric oxide (CuO) nanoparticles suspended in 2,3,3,3-tetrafluoropropene (R1234yf) are investigated in this study. The Pak and Cho model was adopted to examine the densities of nano-refrigerants as the volume concentration of particles in the base refrigerant and temperature varies from 1% to 5% and 273 to 323 K respectively. The models are validated using the experimental studies conducted by various researchers on different nano-refrigerants. The analysis results indicated that the liquid and vapour densities of CuO/R1234yf nano-refrigerant are 10.3% and 62.93% greater than that of Al2O3/R1234yf at 5% particle concentration. From the liquid and vapour density point of view, the CuO/R1234yf nano-refrigerant is superior over Al2O3/R1234yf and TiO2/R1234yf. Results also indicated that at 308 K the liquid phase density of CuO added with R1234yf nano-refrigerant is higher by 12.99% and 8.65% than R134a and R141b respectively. Hence CuO/R1234yf significantly enhances the performance of refrigeration systems.

Nonequilibrium heat and mass transfer processes through liquid-vapor interfaces are studied through solutions of the Enskog-Vlasov (EV) equation and the corresponding system of EV26 moment equations. These models fully resolve liquid and vapor bulk regions and the diffuse interface connecting both. With that, evaporation and heat transfer processes can be studied without the need of modeled interface relations. Comparison of numerical results shows qualitative agreement of moment simulations with DSMC solutions of the EV equation, but quantitative differences. Interface resistivities for jump interface conditions are determined from the simulations, which show marked differences to those found from classical kinetic theory, where dimensionless resistivities are constants. In contrast, the EV models give temperature dependent resistivities, some negative off-diagonal resistivities, and indicate non-linear behavior where resistivities depend on mass and heat fluxes through the interface. In summary, the results point to the urgent need for systematic evaluation of resistivities over a wide range of conditions between weak and strong nonequilibrium, close to and far from the critical point.

Assessment of a novel defrost method for PV/T system assisted sustainable refrigeration system

To reduce the expected increase in global mean temperature, the substitution of R134a as working fluid of refrigeration and air conditioning systems using refrigerants with the lowest carbon footprint possible is a priority. In this paper, the use of R1234ze(E) as pure fluid and mixed with R134a (resulting R450A) is assessed from a thermodynamic point of view. Then, these results are used to support experimental data obtained from a vapor compression system at three evaporating temperatures combined with other condensing temperatures. Regarding refrigerant main thermodynamic properties, R450A is very similar to R134a, even though the HFO R1234ze(E) is the main component of the mixture (58% compared to 42%). For both alternatives, the major differences with R134a are observed in relative molar mass, vapor density and liquid thermal conductivity. Compared with R134a, average cooling capacity obtained is lower for both alternatives (until 34% for R1234ze(E)) due to a reduction in suction density and refrigerating effect. Resulting COP of R1234ze(E) is also slightly lower even though specific compression work is significantly lower. Average R450A COP can match that of R134a at the same compression pressure ratios. Attending to the results of the drop-in tests, the main conclusion of the paper is that only R450A can be used as drop-in replacement if light reduction in system cooling capacity can be accepted.

The process of heat and mass transfer between the air flow and the natural source of cold - ice is considered. It includes heat exchange by sensible heat and moisture exchange through latent heat transfer. This type of heat and mass transfer is observed in seasonal refrigeration storages for air conditioning systems. Snow or ice frozen during the cold season is used as a natural source of cold. The experimental setup created conditions for the contact of the flow of thermal air and natural sources of cold. The air flow is uniform. For this, the setup was carried out. Based on the analysis of experimental data, equations were obtained that determine the process of heat and moisture exchange. These are the dependences of the thermal Nusselt number on the Reynolds number and the diffusion Nusselt number on the Reynolds number. The heat transfer equations were compared with the previously obtained equations for non-uniform air flow in the experimental setup. Heat transfer with a uniform air flow is less intense at high Reynolds numbers from the considered interval. However, at lower air flow rates, the thermal Nusselt numbers become higher than at nonuniform air flow. These dependencies make it possible to calculate the dimensions of the required surface of contact of air with a natural source of cold for designing seasonal refrigeration storages.KeywordsCold accumulationHeat exchange (transfer)Moisture exchangeConditioningThermodynamic parameters

Use of single air extraction and injection to thermodynamically balance the combined heat and mass transfer process

The current electric refrigeration and air condition systems are major causes for ozone depletion and global warming. These systems consume large percent of the worldwide gross production of electricity (around 17%). Therefore, developing new refrigeration systems that are able to work using renewable sources (solar, geothermal, etc) and waste heat sources is necessary to address these problems. In this paper, an experimental investigation and heat transfer analysis of an innovative thermal-mechanical refrigeration (TMR) system are presented. The TMR system replaces the electric compressor of the conventional refrigeration systems with an expander-compressor unit (ECU). The ECU can be driven by ultra-low temperature heat sources, has simple configuration, and high flexibility for the operating conditions. A hybrid electric-compressor and ECU refrigeration experimental setup was built to investigate the performance of the ECU and compare it to the electric compressor with R134a as the working fluid. The effect of the major operating parameters (condenser and evaporator pressures and temperatures) on the performance of the electric and TMR systems are investigated and analyzed. The results show that a maximum COP is obtained at refrigerant mass of 30g in electric mode and at refrigerant mass of 60g in ECU mode. A comparison of the TMR to other existing thermal refrigeration systems showed that most of these systems have more complex configurations, lower COP, and higher capital costs.

Studying the heat and mass transfer process of liquid desiccant for dehumidification and cooling

The results of experimental studies of hydraulic resistance, heat and mass transfer characteristics of a prismatic lattice polymer regular packing for various options of its installation in space are presented. Based on the conducted experimental studies, the most rational design of packing installation was determined (taking into account the decrease in hydraulic resistance and the increase in the efficiency of heat and mass transfer processes), for which the geometric characteristics are given and empirical dependences are obtained, which make it possible to determine the value of the hydraulic resistance of the dry and irrigated packing, the values of the volumetric coefficients of mass transfer and heat transfer. The results of the study can be used in enterprises where different types of cooling towers or devices for conducting heat and (or) mass transfer processes, such as absorption, are used, as well as in the development of packed scrubbers.

A molecular dynamics study on the supercritical evaporation of liquid ammonia under forced convection conditions

This article describes the advantages of the widespread using of the evaporative coolers for cooling recycled wastewater in the air conditioning systems in various industries. The mathematical model of heat and mass transfer processes in the evaporative coolers has been developed. The dependence of the changes in the temperature of the air leaving the evaporator on the irrigation density is considered. When comparing the values obtained by using mathematical expressions and experiments, the resulting difference was 6.7%.