Study of the Influence of Turbulence Models on Hydrodynamic and Thermal Parameters of Heat Carriers in Calculations of Heat Exchangers

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To improve the characteristics of heat and power equipment, it is necessary to develop new designs of heat exchangers, increase the efficiency of heat exchange surfaces, apply modern approaches to the design and calculation of heat exchangers, and create new technologies for their production. The paper proposes modern compact coil heat exchangers of the in pipe type with a variable bending radius of the helical spiral and the engineering method calculating the thermohydrodynamic characteristics of such devices. The calculation of a coilheat exchanger with a linearly varying bending radius of a helical spiral (cone-shaped coil heat exchanger) has been performed. They performed the experimental study of heat transfer in a cone-shaped coil heat exchanger of the pipe-in-pipe type. They obtained the empirical formulas for the Nusselt number and hydraulic resistance coefficient. It has been established and experimentally confirmed that cone-shaped coil heat exchangers are morecompact and efficient in comparison with the known cylindrical coils. It is proposed to modernize the hot water preparation unit in one of the residential buildings in the city of Kazan (Russia) by replacing the outdated shelland-tube heat exchanger with a compact cone-shaped coil heat exchanger, which made it possible to reduce the duration of hot water preparation significantly, to reduce the costs of the heating medium and, thus, to reduceconsumer costs.

There are the various types of heat exchangers. The selection of the heat exchanger right type is the first basic assumption for its optimal operation. The heat exchanger calculation itself is another prerequisite for its optimal operation. This article deals with the variables which are usually used to describe the stationary operation of any recuperative heat exchanger with two incoming and two outgoing streams. The knowledge of these variables, including the facts resulting from them, is necessary not only from the point of view of the calculation but also from the point of view of the evaluation of the experimental data of any heat exchanger. The variables values needed for the calculation of heat exchangers, so-called key variables, must always fall within the values range determined on the basis of generally valid knowledge about heat exchangers. The article also deals with the determination of the limit values defining the values range of these key variables.

Numerical correlation for the pressure drop in Stirling engine heat exchangers

In the case when there is a difference in consumption on the side of the cold and hot heat carriers, the use of channels with the same corrugation height in one heat exchanger leads to a decrease in velocity of the heat carrier on the side with low consumption. Low velocity contributes to appearance of deposits on the heat transfer surface, which leads to disruption of operation mode of the apparatus and a forced clean-up stop. In case of using channels with a different corrugation height (cross-section area), velocities in the channels are aligned and intensity of emergence and growth of contamination falls sharply. It also allows us to reduce the heat transfer surface area of the apparatus and fully implement permissible pressure losses on the sides of the heat exchanger. For designing of heat exchangers of this structure, the authors developed the mathematical model that allows making calculations of heat exchangers for assigned operation conditions with the use of geometrical data of the plates, thermophysical properties of heat carriers and criterial equations for plates of the selected type. The calculation algorithm involves determining of the ratio between corrugation heights. Practical value lies in the fact that the proposed approach makes it possible to extend the service life of the heat exchanger prior to a maintenance stop. This enables provision of continuity of the technological process and decreases operation costs. Calculation of the heat exchanger of hot water supply by the parallel scheme of attachment to heating networks and the heat exchanger of stage 1 of the two-stage hybrid scheme was presented. Calculation data indicate a decrease in the heat transfer surface area compared to heat exchangers with channels of equal height, full realization of pressure losses and their alignment in the channels, which facilitates an increase in resistance to contamination of the plates’ surface. Calculation showed the advantage of using apparatuses with channels of different cross-section area. The higher the ratio between consumption of heat carriers in the channels, the stronger this advantage.

The article discusses the development of an ontology for the subject area using a shell-and-tube heat exchanger as an example. This ontology enables the selection of the type of heat exchanger (fixed tube sheets, compensator, U-shaped tubes) based on the coolant (ammonia, methanol), heat exchange process conditions (pressure, temperature), and the geometric parameters of the heat exchanger (device diameter). The ontology is designed for use in the hardware design phase of chemical-engineering systems. A functional model is presented that outlines the key stages and information flows in the hardware design of chemical-engineering systems. Each design stage utilises an information model, converting the input information flow into an output one. The article describes an information model for selecting the type of heat exchanger, represented by production rules, which includes operators for determining the material of the heat exchanger elements based on the coolant, material-specific design, and the type of heat exchanger. A prototype of the described information model is implemented in the Protégé ontology editor. The ontology and an example query for determining the type of heat exchanger for a given coolant and heat exchange process parameters are provided. The data for creating the ontology are sourced from regulatory design documents. The article concludes that an ontological approach is feasible for creating "smart" design documents, including standards and technical specifications comprehensible to both humans and computers.

The objective of the present work is to study the influence of three turbulence models and find out the suitability of the k-ω SST turbulence model. The thermal performance of a heat exchanger with single continuous helical baffle is studied numerically and experimentally. A commercial CFD package (Ansys-Fluent),based on the finite volume method, is used to predict the performance of 25° continuous helical baffled Heat Exchanger using 3 turbulence models viz. k-ε Realizable, k-ε RNG and k-ω SST. The experiments are carried out in counter-flow mode with cold fluid (ambient water) on the shell side and hot fluid (hot water) on the tube side. In the experimentation, the variation of the shell side heat transfer coefficient and of the pressure drop with shell side mass flow rate is tested. For the validation of the numerical model, the numerical results are compared with the experimental results and with the published literature. The experimental heat transfer coefficient exhibits a deviation from the correlated values in the range of 1.5% to 5.3%, whereas pressure drop shows deviation in the range of 4.22 to 6.67%. The results indicate that all the three turbulence models are in good agreement with the experimental findings. The k-ε Realizable model is the best turbulence model for the prediction of heat transfer and k-ω SST is the best turbulence model for prediction of pressure drop. For the overall performance, k-ω SST model is suitable for the prediction of both heat transfer and pressure drop and it helps in capturing the swirling flows in helical baffle heat exchangers.

The article is devoted to the study of the hydrodynamics of heat carriers in heat exchangers. Giventhe widespread use of this device, increasing its thermal and hydraulic characteristics has become so important for theirdesigners. In the work, the calculation of non-stationary flows of liquids through the heat exchanger tubes is carried out.Water ("hot" coolant) and oil ("cold" coolant) are used as heat carriers, between which heat exchange occurs through thesolid surface of the pipeline, which is the boundary between the heat carriers. When calculating the movement of oil, thefact was taken into account that the dynamic viscosity depends very much on temperature and, depending on temperature,the Reynolds numbers also change. And, as a consequence, when flowing through a sufficiently thin channel, the flow regimechanges, namely, the transition from the laminar flow regime to the turbulent one is manifested, while this effect is notobserved with the analytical calculation method for constant viscosity. The Reynolds-averaged Navier-Stokes equations,closed using the turbulence model, were used for numerical modeling of fluid dynamics of heat carriers. Verification of theheat exchanger calculations was carried out in the software package "Ansys Fluent".

New analytical-numerical method for modelling of tube cross-flow heat exchangers with complex flow systems

Three different geometries have been used in this study, which investigated the effect of inclination angle on convective heat transfer in corrugated channels. The corrugated channel geometries used have inclination angles of 30°, 45°, and 60°. The finite volume method has been used in the study with numerical methods. k-ω, SST, and transition SST have been used as turbulence models. The mesh file used in the analysis consists of 192000 cells. The results obtained from the calculations using a corrugated channel with a 30° inclination angle have been compared with the experimental data. As a result of this comparison, the turbulence model to be used for this study has been determined as SST. SST turbulence model has been used in the analyzes made using a corrugated channel with 45° and 60° inclination angles and the results have been compared with each other. The Reynolds number ranges from 2225 to 7380. The Nusselt number and friction factor have been calculated using the data obtained as a result of the analysis. Based on the calculated values, it has been determined that the heat transfer increases as the angle of inclination increases in the corrugated channels. It has been also observed that the Nusselt number increased as the Reynolds number increased. It has been also seen that the analyzes for high Reynolds number gave results closer to the experimental data.

Introduction. In modern conditions (depletion of deposits, complication of development conditions, the application of sanctions on the import of equipment, etc.), a special role is played by increasing the resource of equipment used in the mining industry, including processing drilling impact-rotary tools. One of these methods is cryogenic processing of machining tools, as a result of which the hardness and wear resistance of the cutting tool increase several times. For the production of cryogenic substances such as nitrogen, oxygen, methane, etc. in the liquid state, a method is proposed based on the use of adsorption-cryogenic technology implemented through the use of heat exchangers, the development of calculation methods of which is the subject of this article. The purpose of the research is to develop a methodology for calculating heat exchangers used as cold sources for cryogenic surface treatment of mining equipment tools in order to harden it. Research methods. The analysis of the current state of development of technical and technological solutions in the field of development and construction of heat exchangers was carried out through the use of such informal methods as: expert, morphological and factographic. To optimize the modeling calculations of the a priori selected configuration of the heat exchange surface, the method of uniform (blind) search was used. Research results and discussion: 1. In the course of the conducted research, the results of the analysis of the current state of development of technical and technological solutions in the field of development and construction of heat exchangers were obtained. 2. A method for calculating twisted heat exchangers has been developed. 3. It is determined that the presented method of calculating heat exchangers used as cold sources for cryogenic surface treatment of mining equipment tools in order to harden them allows optimization of modeling calculations a priori of the selected configuration of the heat exchange surface. 4. It is established that: heat exchangers with finned pipes should be used when the ratio of the heat transfer coefficient of the flow inside the pipes to the corresponding coefficient in the inter–tube space is 3 ...5 times greater. 5. The use of finned pipes in heat exchangers makes it possible to increase the area of the outer surface of the pipes due to finning, thereby improving the thermal characteristics of heat exchangers, while reducing its weight and size characteristics. 6. It has been revealed that heat exchangers with dense winding are less efficient compared to devices with sparse winding, and heat exchangers with smooth pipes should be used in cases of proximity of the values of the heat transfer coefficient of flows in the tube and inter-tube space. Resume: 1. The analysis of the current state of development of technical and technological solutions in the field of development and construction of heat exchangers was carried out. 2. A method has been developed for calculating heat exchangers used as cold sources for cryogenic surface treatment of mining equipment tools in order to harden them. 3. The developed method of calculating heat exchangers can be used to design heat exchangers consisting of pipes with different surface geometries that are wound onto the core in a spiral, used both in the extractive industries and in the engineering, chemical, oil and gas, rocket and space industries. The further development of the research topic is the optimization of the operating parameters of the heat exchanger according to the criterion of energy efficiency.

The problem of intensification of thermal processes and the process of heat exchange in a plate heat exchanger of the HHN02 type is considered. A comparison is made of two types of heat exchangers - shell-and-tube and plate. The formulas for calculating heat transfer and heat transfer coefficients are presented. The analysis of theoretical studies on the intensification of heat transfer in a plate heat exchanger with a corrugated surface of the plates is carried out. A full-scale experiment was carried out on a specially designed semi-industrial stand “Independent heating system of a residential building”. It has been found that the use of a corrugated surface is capable of providing a high value of the heat transfer coefficient. It has thus confirmed the advantages of the plate heat exchanger in comparison with the shell-and-tube.

Heat exchangers are devices whose primary responsibility is to transfer heat, typically from one fluid to another. In such applications, the heat exchangers can be parallel flow, crossflow, or counter flow. An essential part of any heat exchanger analysis is the determination of the effectiveness of the heat exchanger. In the present work, three different types of heat exchangers are investigated. Numerical and experimental performance analyses are applied. The main objective of the present work is to compare the effectiveness of each heat exchanger at different conditions. Six experimental investigations for Plate, shell & tube, and fluidized bed heat exchangers are executed. All experimental tests are reached to steady-state conditions. The results show that the counter flow plate heat exchanger has an effectiveness of 90% compared with the parallel flow of 60% effectiveness for working experimental conditions. Also, the fouling effect in decreasing heat transfer is cleared. In the present work, fouling decreases effectiveness from about 18% to about 4%. In addition, the effectiveness of the fluidized bed heat exchanger depends on the material used for the bed. Finally, the overall heat transfer coefficient is obtained and compared for all experimental tests, and it is directly proportional to the effectiveness of the heat exchanger. The FEHT program is used to get the temperature distribution in all types of present work heat exchangers.

Two-phase microfluidic cooling has the potential to achieve low thermal resistances with relatively small pumping power requirements compared to single-phase heat exchanger technology. Two-phase cooling systems face practical challenges however, due to the instabilities, large pressure drop, and dry-out potential associated with the vapor phase. Our past work demonstrated that a novel vapor-venting membrane attached to a silicon microchannel heat exchanger can reduce the pressure drop for two-phase convection. This work develops two different types of vapor-venting copper heat exchangers with integrated hydrophobic PTFE membranes and attached thermocouples to quantify the thermal resistance and pressure-drop improvement over a non-venting control. The first type of heat exchanger, consisting of a PTFE phase separation membrane and a 170 micron thick carbon-fiber support membrane, shows no improvement in the thermal resistance and pressure drop. The results suggest that condensation and leakage into the carbon-fiber membrane suppresses venting and results in poor device performance. The second type of heat exchanger, which evacuates any liquid water on the vapor side of the PTFE membrane using 200 ml/min of air, reduces the thermal resistance by almost 35% in the single-phase regime in comparison. This work shows that water management, mechanical and surface properties of the membrane as well as its attachment and support within the heat exchanger are all key elements of the design of vapor-venting heat exchangers.

In this study, heating performances of horizontal and vertical earth-air heat exchangers with equal pipe lengths were evaluated in ANSYS FLUENT 19.2 simulation program based on CFD for the winter period of Bitlis, Turkey. Thermal per?formance analyses regarding vertical and horizontal 3-D-modeled earth-air heat exchangers were evaluated in steady-state condition of the simulation program for different values of Re = 5?103, 104, 2?104, 4?104, 6?104, 8?104, and 105 numbers by using standard k-? turbulence model. Numerical results obtained from CFD based simulation program were compared with a numerical study in the literature, and it was determined that there was a consistency between the results. The pressure loss and fan power values of horizontal and vertical earth-air heat exchangers were also investigated in addition their thermal performances. A good agreement was found between the pressure loss values obtained from the theoretical and simulation calculations of both earth-air heat exchangers. Considering the temperature increases in both earth-air heat exchangers, the highest and lowest temperature increases were observed in vertical earth-air heat exchanger with 22.52 K and 10.67 K, respectively. The best thermal performance was observed in vertical earth-air heat exchanger for 5?103, 104, and 2?104 values of Reynolds number and in horizontal earth-air heat exchanger for 4?104, 6?104, 8?104, and 105 values of Reynolds number.