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The article is devoted to the fast heat transfer processes taking place during permanent joint formation by MAG welding. It is shown that the conditions of heating and crystallization of molten metal accompanying the permanent joint formation are extensively affected by the welding method, quantitative characteristics of mass transfer and energy parameters of the welding regime. The above mentioned combination of technological indicators is directly bound with the thermal welding cycles that linearly depend on the rate of the energy input required for the permanent joint formation. [ABSTRACT FROM AUTHOR]

The behavior of supercritical fluids is very different from normal fluids particularly in terms of its properties which has beneficial effects for several applications. Thus, hydrothermal gasification/liquefaction of biomass at near and above supercritical conditions is rapidly gaining its importance due to less/no evaporation losses of water and the solvent effects of water at such conditions. Though a lot of research has been done on heat transfer studies in supercritical fluids typically for cooling nuclear power plants very limited number of studies are available for supercritical water gasification of biomass. Several empirical correlations were reported in the literature for predicting the heat transfer coefficients. In this study, we have particularly investigated different correlations like Dittus-Boelter, Sieder and Tate and Krasnoshchekov-Jackson which are then validated with the experimental data using different fluids (air, CO2 and water) and conditions with bench and pilot scale facilities. Though, heat exchange behavior with different fluids was studied herein we only focus on the supercritical water. [ABSTRACT FROM AUTHOR]

The operation efficiency of air conditioning systems (ACS) depends on a duration of their operation period and loadings of their refrigeration machines of definite installed (design) refrigeration capacity. The longer duration of ACS operation, the larger refrigeration capacity production according to current cooling needs during operation period. So as the operation efficiency of railway ACS and their refrigeration machines depends on a duration of their operation on the route line it can be estimated by a refrigeration capacity production for the operation period considered. The method to determine the rational design heat load on railway air conditioning system matching the changeable climatic conditions and providing a maximum refrigeration capacity production for the operation period was developed. One of the most attractive reserves of enhancing the efficiency of air conditioning systems consists in efficient operation of their air coolers (refrigerant evaporators), intended to match varying thermal loads in response to current changeable climatic conditions. A proposed concept of enhancing heat efficiency of air coolers with boiling refrigerants inside channels is intended to solve the problem of uneven external thermal load distribution between refrigerant coils and changeable current thermal loads on air coolers by over filling all the coils through liquid refrigerant jet pump (injector) recirculation that provides excluding the final stage of refrigerant evaporation with low intensity of heat transfer to superheated vapors. The principle of over filling all the refrigerant coils by liquid refrigerant injector recirculation is very useful for air coolers of railway air conditioners influenced by varying heat loads during operation at current changeable climatic conditions on the routes. [ABSTRACT FROM AUTHOR]

The effect of the water operating pressure on the external heat losses value of the shell-and-tube heat exchanger by using the heat loss recovery systems based on the Rankine steam cycle has been analyzed in the present paper. A comparative analysis of the effectiveness of insulating various sections of the outer surface of the heat exchanger casing was also carried out. The choice of the heat exchanger was determined by the design simplicity, the ability to withstand high pressures and temperatures, as well as by the fact that its design makes it easier to carry out thermal insulation. The analysis of the heat transfer processes in regenerative HE was conducted on the basis of the equations contained in the ANSYS Fluent package. When modeling heat exchange processes, the following boundary conditions have been adopted: the hot air temperature at the inlet Th is equal to 600 °C (the first option) and 800 °C (the second option); the water temperature at the inlet Tw is 16 °C; the ambient temperature Ta is 20 °C. The heat transfer coefficient α between the environment and outer surface of the HE is considered as constant. The obtained results revealed that heat losses significantly decrease only when the first section is insulated in 25% of the casing surface. After isolating 50% of the casing surface, the heat losses amount remained largely unchanged. It can also be concluded that with an increase in excess water pressure, the heat losses value decreases from ∼40% to ∼8% (at the inlet gas temperature of 800 °C) and from ∼34% to ∼7% (at the inlet gas temperature of 600 °C), when the casing surface insulating degree changes from 0% to 100% and water pressure varies from 1 to 150 atmospheres, correspondingly. This is due to the fact that the water pressure increase results in increasing the boiling point and the liquid water heating area with a high heat capacity. The latter increases the heat flux perceived by the water from the hot gas, and therefore leads to the external heat loss decrease. [ABSTRACT FROM AUTHOR]

This article describes the task of intensifying heat transfer in heat exchangers for microturbines using additive manufacturing technologies. A review of various types of heat exchangers is done, as well as methods for their manufacturing based on additive technologies are considered. Numbers of shell-and-tube heat exchanger models are proposed with various configurations of tube finning, which are made using selective laser sintering. In order to verify the efficiency of the proposed heat exchangers, an experimental stand and an experimental technique have been developed. Based on the experimental data, dimensionless similarity criteria are calculated - Reynolds and Nusselt's numbers, heat transfer coefficients for the heat transfer process in the models of heat exchangers. From the analysis of the obtained data, a conclusion was drawn about an increase in the heat transfer intensity in 1.5 times by introducing internal fins in prototypes of shell-and-tube heat exchangers. [ABSTRACT FROM AUTHOR]

Increasing the energy efficiency of HVAC systems for rooms in chemical and petrochemical industries during their temperature control is an important problem. A mathematical model is proposed for determining the values of indoor air temperatures and surface temperatures of envelopes in order to calculate the predicted values of heat flows in building and structures. An experimental research showing the relevance of the proposed methodology was conducted. [ABSTRACT FROM AUTHOR]

The use of ice slurry in the food industry is becoming more common, and this trend will continue. First of all, this is due to its high cooling ability, which allows to lower the temperature of the product in a short time period. The technology for the production of ice slurry at a pressure below the triple point of water by the method of drip dispersion has the advantage because of increasing the heat transfer surface during its atomization and direct cooling. Based on a mathematical model that allows evaluating the change in droplet mass during crystallization, an analysis was made of the freezing of droplets in a spraying full cone for various operating modes of nozzle. The effects of mediums pressure on the amount of ice produced and water vapor discharged and the effect of the average droplet diameter on the power consumption of the vacuum pump were evaluated. This analysis may be useful in designing a vacuum-evaporation plant for the production of ice slurry. [ABSTRACT FROM AUTHOR]

The model of equivalent channels for the heat storage nozzle of a regenerative heat exchanger is considered. Differential heat transfer equations between the heat carrier flow and the surface of the equivalent channel are given. It is suggested to evaluate the efficiency of heat transfer by the coefficients of accumulation and heat recovery of ventilation flows. Criteria for calculating the heat transfer coefficient under laminar and transient modes of gas motion in an equivalent channel is given. The results of a computational study are presented, which make it possible to evaluate the effects of air velocity in the channel on the efficiency of a regenerative heat exchanger. [ABSTRACT FROM AUTHOR]