Intensification Of Heat Transfer Processes Research Articles

Experimental thermal performance intensification of gravitational water vortex heat exchanger using hexagonal boron nitride-water nanofluid

One of the main concerns presently is the rapid improvement of thermal technology for widespread heat exchanger applications involving energy conservation and intensification of heat transfer process. For the enhancement of heat exchange processes, nanofluids have been considered as a potential substitute for conventional fluids, which in general have subpar thermal characteristics. Therefore, the present study focusses on the heat transfer enhancement and the intensification of overall thermal performance of the developed gravitational water vortex heat exchanger (GWVHE). The experimental investigation first involves the preparation and characterization of water-based hexagonal-boron nitrides (h-BN) nanofluids as well as the computation of thermophysical properties of different volume fractions of h-BN nanofluids. Subsequently, a comparative analysis evaluating the thermal energy exchange capabilities of water-to-water and water-to-h-BN nanofluids combination of two fluids has been conducted to investigate the intensification in thermal performance of the developed GWVHE. The experimental investigation is primarily based on calculating the thermal energy balance, overall heat transfer coefficient, log-mean temperature difference (LMTD) and the effectiveness of the developed GWVHE using effectiveness-NTU method. The experimental results reported that 0.02 % volume fraction of water-based h-BN nanofluids is more suitable for further testing of the developed GWVHE to mitigate the stability concerns at higher concentrations. The minimal thermal losses, that lie within the 10 % confirms the thermal energy balance and by using the h-BN nanofluids −to-water strategy for two fluids. The maximum value of heat exchange rate significantly increased from 8490 W to 9998 W with the utilization of h-BN nanofluids-to-water as compared to water-to-water combination. Furthermore, the maximum values of LMTD employing h-BN nanofluids are found to be reduced from 21.15 K to 17.53 K compared to the water-to-water combination confirming the intensification in thermal performance of GWVHE. The overall heat transfer coefficient values are also found higher when h-BN nanofluids are substituted instead of water. The maximum value of overall heat transfer coefficient for h-BN nanofluids-to-water combination is improved from 874 W/m2K to 1142 W/m2K compared to water-to-water combination. Moreover, by utilizing the h-BN nanofluids-to-water combination, the values of effectiveness are intensified by an average of 13.5 % and 9 % in both sets of testing parameters, respectively. All these maximum improvement in findings for heat exchange, overall heat transfer coefficient, effectiveness, and reduction in LMTD values are obtained by maintaining the inlet mass flow rate of cold fluid at 0.142 kg/s, while the inlet mass flow rate of hot fluid was varied in the range from 0.092 kg/s to 0.133 kg/s. Additionally, the inlet temperatures of hot and cold fluids are kept at 330 K and 296 K, respectively. Hence, the utilization of h-BN nanofluids is proven to be highly effective for boosting thermal energy exchange and enhancing the overall thermal performance of the developed GWVHE.

Effect of fluid properties on intensification of heat transfer process in oil-water droplet flow in a T-shaped microchannel

Droplet-based microfluidics refers to the production and utilization of micrometer-sized droplets. This technology has found widespread applications in drug delivery, biotechnology, disease prevention, chemical analysis, cell research, and functional material synthesis. Cooling of high heat flux electronic systems and hot spots is another application of the droplet flow which is focused by the current work. In this paper, formation and hydrothermal performance of oil droplets in water flow in a T-junction microchannel is studied by a 3D numerical simulation. Effects of different parameters such as surface tension, wettability and heat flux on the droplet size, heat transfer rate and pressure drop are investigated. It is observed that the shear between the discrete and continuous phases results in recirculating zones in both phases which contribute to improved mixing and heat transfer rates. Liquid film thickness adjacent to the heated surface and the droplet velocity are two other factors that positively affect the heat transfer. Decreasing the surface tension is shown to enhance the heat transfer rate. Moreover, an optimum value is found for the contact angle as 135° at which a 23 % heat transfer enhancement is achieved. Results also indicate that droplet size decreases with increase of contact angle while it slightly increases with increase of the surface tension and negligibly varies with the applied heat flux.

Experimental comparison of finned tube heat exchangers for heat rejection under natural convection conditions

This article presents experimental analysis of heat removal capabilities of a wavy finned tube heat exchanger operating under natural convection conditions Two types of heat exchangers are considered in this work: standard construction exchangers, most often found in waste heat rejection devices and exchangers based on a novel geometry, which supports intensification of heat transfer processes under natural convection. The aim of the research was to experimentally verify the novel conception and to determine the thermal efficiency of heat exchangers under the influence of selected design parameters. The factors analyzed in the research include: temperature difference between the heat transfer surface and the surrounding air, different angles of the units transferring heat and the height of the exchanger casing. Four variants of the casing height were considered in this work. The conducted research also allowed to determine the influence of the fin spacing on the performance of the heat exchangers under natural convection conditions. The test results obtained show that the novel geometry allows for significance improvement of heat transfer efficiency. The proposed solution allowed to increase the density of the heat flux from 12% in (fin spacing of 2.8 mm) up to 24% (fin spacing of 5.6 mm). It was also observed that the height of the casing has significant impact on the heat transfer efficiency. For each of the analyzed cases, the heat exchanger without a casing was characterized by the lowest thermal efficiency, while the highest heat flux was removed from the exchanger with the highest casing (up to four times higher heat transfer rate). It was determined that the largest amount of heat was removed from exchangers located horizontally, while the exchanger with an angle of 60°, was characterized by the lowest rate of heat removal. Another factor which was determined to have significant impact on the performance of the heat exchangers is the spacing between the fins. The research results obtained shows that increasing the spacing of the finned surface from 2.8 mm to 5.6 mm resulted in an average twofold increase in thermal efficiency in the case of four-row exchangers and an almost threefold increase in the case of two-row exchangers. The obtained results can be used as basic guide for the development of finned tube heat exchangers operating under natural convection conditions.

CHOICE OF THE PARAMETERS OF TWISTED TAPES FOR HEAT TRANSFER INTENSIFICATION IN THE CHANNELS OF THE POWER EQUIPMENT COOLING SYSTEM

The paper presents the results of analyzing the efficiency of heat removal processes in the channels of the power equipment cooling system. The processes of heat transfer in the ventilation ducts of the cooling system for traction motors of diesel locomotive power plants are considered. The feasibility of using artificial intensification of heat transfer processes in the ducts with the help of spiral tapes is analyzed. Convective heat transfer intensifiers of this type are characterized by a significant increase in heat transfer coefficients, ease of manufacture, and low material consumption. Such devices can be used both in the design of new units and in the modernization of existing heat exchange equipment. The efficiency of intensification of heat transfer processes was evaluated by the level of reduction in the power of cooling system fans. The estimates were made assuming the same heat transfer coefficients in the channels with spiral strip inserts and in the channels without inserts. The length of the spiral tapes and the channel is the same. The effect of finning of the channel surface by the inserts was not taken into account. Known criterion equations were used to calculate the convective heat transfer coefficients and hydraulic resistance. It is shown that, despite the increase in hydraulic resistance coefficients and pressure losses in the channels of the air cooling system, the use of spiral tapes due to a noticeable decrease in the speed and, consequently, the cooling air consumption provides a reduction in fan power in a wide range of geometric parameters of spiral tapes while ensuring the required temperatures of equipment elements. The influence of the temperature of the walls of the cooling system channel on the required fan power when using artificial heat transfer intensifiers in the form of spiral tapes is estimated. Recommendations for the selection of geometric characteristics of spiral tapes are proposed, at which, taking into account the values of the channel wall temperature, a decrease in the power of the cooling system fans should be expected.

Determination of the Specific Heat Flux During Boiling of the Dispersed Phase of the Emulsion

The intensity of heat exchange between the boiling emulsion and the enclosing surfaces is associated with the physical phenomena of the formation, growth, and destruction of vapour bubbles of the low-boiling component in the liquid phase. This article presents a methodology to assess the intensity of heat exchange processes. Using this technique, it is possible to predict the energy parameters of heat exchange equipment and the degree of intensification of heat transfer processes.

Influence of gas burner design on heat transfer in low-power furnaces

An experimental study showed that the influence of these factors is mutually equalized and the heat transfer in the combustion chamber does not change significantly. It was possible to establish that the nature of the heat absorption of individual elements of the combustion chamber did not change significantly in comparison with experiments with kinetic fuel combustion and distributed injection. An increase in heat removal can be obtained by no more than 5-7 % with an excess air coefficient of up to 1.5. It has been established that the use of an infrared burner provides an intensification of heat transfer processes in the furnace by 5-9 % only when operating on gas with an excess air coefficient of up to 1.25. It has been established that the maximum possible (subject to the operating conditions of the furnace) intensification of heat transfer in the studied range of geometric and operating parameters of the furnace is up to 15 %. Significantly turbulize and radically change the aerodynamics in the furnace volume allows the reverse scheme of the development of the torch and the movement of combustion products.

The influence of microstructured surfaces and liquid pulsations on increasing heat transfer in a minichannel

The paper compares two methods for improving the intensification of heat transfer during boiling in a rectangular minichannel. Studies were carried out of the influence of additive structures on the crisis of heat transfer in a minichannel with a two-phase flow, as well as studies of the influence of high-frequency pulsations of liquid flow on the intensification of heat transfer processes during flow boiling.

INFLUENCE OF VARIOUS FACTORS ON THE HEAT TRANSFER CHARACTERISTICS OF MINIATURE TWO-PHASE THERMOSYPHONS WITH NANOFLUIDS

Currently, various types of nanofluids are of increasing interest as heat carriers for heat transfer in thermosiphons and other evaporative-condensation devices. This paper presents and analyzes experimental data on heat transfer characteristics (total thermal resistances, maximum transferable heat fluxes and equivalent thermal conductivity) of two-phase miniature thermosyphons with nanofluids. Geometric parameters of thermosiphons for all experimental samples were identical and were: total length 700 mm, inner diameter 5 mm. The length of the heating zone was changed stepwise from 45 mm to 200 mm. The length of the condensation zone was 200 mm for all investigated thermosyphons. The amount of coolant in the thermosiphons was the same, and its height in the heating zone before the start of the study was 88 mm. Distilled water and aqueous nanofluids with nanoparticles of carbon nanotubes, synthetic diamond, and carbon black were used as heat carriers. The main attention is paid to the study of the influence of the filling factor and the angle of inclination of the thermosyphon, the value of the transferred heat flux and the chemical nature of the coolant (nanofluid) on the heat transfer characteristics of thermosyphons. The strong influence of these factors on the efficiency of a miniature closed two-phase thermosyphon has been demonstrated. A more than twofold increase in the heat transfer characteristics of thermosyphons (the maximal transferred heat flux) was obtained with a sharp decrease in their thermal resistance. It is assumed that the significantly higher heat transfer capacity of such thermosiphons compared to those filled with water is explained not only by the higher thermal conductivity of the coolant, but also by the appearance of a peculiar porous structure that prevents the appearance of a vapor film and promotes the intensification of heat transfer processes during boiling. Bibl. 16, Fig. 10, Tab. 2.

Modelling steady-state convective heat transfer in different periodic open cellular structures (POCS) – A superposition approach

Additively manufactured periodic open cellular structures (POCS) offer promising attributes for the intensification of heat transfer processes. Their geometry and consequently thermal transport properties are highly customizable resulting in a vast design freedom. To fully exploit their potential, a detailed understanding of the relationship between their geometric and thermal transport properties is crucial. Therefore, in this contribution the convective heat transfer in four different unit cell geometries and varying cell dimensions during fully developed laminar steady-state flow is investigated. Numerical simulations are used to get a detailed insight of the local flow structures and heat flux density distributions. A correlation between the heat transfer coefficient of each unit cell and the heat transfer capability of unconnected strut arrangements with similar flow paths is introduced. On this basis, a new modelling approach is established in combination with the concept of superposition. It takes into account the detailed geometry of the porous medium and shows good agreement with the simulation data of all unit cell geometries studied in this work with porosities above 79.1%. Thus, it is the first model applicable to different types of unit cells.

Scheme of incineration of wood waste in a furnace with a circulating boiling bed

Cyclonic-vortex technology of solid fuel combustion allows to reduce the furnace volume of the boiler unit, dimensions and weight. Mixing of fuel particles and air in a swirling stream provides intensive heat exchange and fuel combustion in a limited furnace volume. The results of improving the thermal scheme of combustion of pulverized fuel (wood waste) in a circulating fluidized bed are presented. The results of numerical calculations showed the possibility of burning sawdust with a particle diameter of 25 μm to 750 μm. When the air-fuel mixture is supplied from the bottom of the furnace, and the secondary air from the top of the furnace, ash is removed in liquid form through a slot in the lower part of the furnace. It was found that with a sawdust moisture content of 10%, ash content Ac = 0.6%, a higher combustion heat of 17.7 MJ / kg, particle entrainment is 19.2 %, the degree of particle burnout is 92.5%, which requires afterburning of entrainment in a circulating boiling layer. One of the ways to improve Ukraine's fuel and energy balance is to use alternative sources of heat to natural gas, namely solid fuels or wood industry waste. It is necessary to involve specialized technologies for burning solid fuels. Currently, there is an incomplete theoretical and practical knowledge about the processes of burning wood waste.The use of natural gas as a source of thermal energy occupies a significant part of Ukraine's fuel and energy balance. To increase the share of solid fuels, it is necessary to study in more detail the processes of their combustion and the use of methods of intensification of heat transfer processes in the furnaces of solid fuel boilers. The results of numerical calculations showed that the use of vortex furnace instead of direct flow in the scheme CKS increases the residence time of particles in the furnace, which reduces the multiplicity of fuel circulation, reduce the size and weight of the furnace

DETERMINATION OF HEAT TRANSFER EFFICIENCY IN THE CONDITIONS OF FORCED CONVECTION FROM PIPES WITH SPECIAL RIBS

Today, heat transfer processes are present in almost all technological processes of various industries. In heat exchange processes, shell-and-tube heat exchangers are quite effective and easy to manufacture, as the long-term practice of using these devices has shown. Therefore, intensification of heat transfer processes, improvement and development of appropriate equipment is a very urgent task. The object of research is a heat-exchange element with special finning on heat-exchange tubes. The subject of research is the heat transfer processes implemented in a heat exchange element with special finning. The aim of the study is to determine the efficiency of heat transfer of the finned surface of the heat exchange element under conditions of forced convection and to evaluate its efficiency by means of experimental and computer research. This article presents a computer simulation that allows to adequately assess the efficiency of using various designs of finning elements of heat exchange equipment. This is confirmed by the convergence of the experimental data and the results of computer simulation (the discrepancy between the results of the experiment and computer simulation does not exceed 5 %). Experimental and computer studies have shown that the proposed technical solution is more effective than standard ones and can be used in the design of new equipment or improvement of the existing one.

Fabrication of a hybrid shell and double pipe heat exchanger by means of design and performance assessment

Herein, a novel hybrid shell and double pipe heat exchanger was fabricated and applied for intensification of heat transfer process. The heat transfer surface was increased using double pipes instead of simple tubes. The streams flowed through the annulus, and exchanged the heat with two fluids in sandwiched manner. The performance of this device was carried out by modeling and simulation. The simulated results were validated with experimental data under steady state environments for different flow patterns, namely, co-current, counter-current, insulated and non-insulated configurations. Heat exchanger worked with three fluids such as two hot fluids and a cold fluid (H-C-H) or vice versa (C-H-C) relying on the purpose of heating or cooling. The results indicated that the predictions of the temperature variations in terms of magnitude and trends were in good agreement with the experimental data. Fluid flow distributions in all regions of the heat exchanger were found to be turbulent. Pressure drop and standard uncertainty values were calculated to be small. It was found that the proposed heat exchanger was more efficient (efficiency ~60%) than other design configurations (e.g., double pipe, shell and tube) as well as the existing lab-scale shell and tube heat exchanger (training apparatus).

Action Planning for Improvement of Design and Operating Parameters of Ball-Bearing Steel Tube Annealing

Annealing process was reviewed for hot rolled 100Cr6 steel tubes in a chamber furnace with subhearth burners. Based on the obtained results of thermotechnical testing at the chamber furnace it was detected that the current annealing curve conceptually do not allow production of satisfactory quality tube in the course of annealing. As a result an adjusted curve corresponding to the annealing requirements for 100Cr6 steel and chamber furnace capabilities was proposed. Causes were determined for poor quality tube annealing and detailed review was carried out for chamber furnace capabilities in terms of required heat treatment duration ensuring sufficiently high quality of annealed tubes. List of actions allowing significant improvement of chamber furnace performance and annealed tube quality was considered. Chamber furnace renovation option was proposed where the total heat treatment time of tubes may be reduced significantly through heat transfer process intensification in the furnace working space, uniformity of tube heating in the batch, annealing quality and therefore furnace performance may be improved.

Heat Transfer in the Ducts of the Cooling Systems of Traction Motors

Requirements for increasing thermal efficiency heat exchangers, which lead to energy saving, material and reduction cost, and as a result of reducing the impact on the environment, led to the development and use of various methods of increasing heat transfer. These methods are called intensification of heat transfer processes. Intensification of heat and mass transfer processes is of great importance for making progress in improving the existing and creation of new energy and heat-exchange equipment. Among the ways of intensifying heat transfer, the swirling of flows of working media is one of the simplest and most common methods and is widely used in energy-intensive channels of nuclear power plants, heat exchangers, aeronautical and rocket and space equipment, chemical industry and other technical devices. We have proposed formulas to determine the cooling air velocity necessary to ensure the required temperature condition of the traction motor assemblies. Decrease in the power of fans in the cooling system using the artificial heat transfer intensification in the ducts was estimated based on the generalization of the results of calculations.

Intensification of Heat Transfer Processes in the Low Temperature Short Heat Pipes with Laval Nozzle Formed Vapour Channel

The results of flow studies of moist vapour in Laval-liked vapour channels of short linear heat pipes (HPs) are presented. The increase in heat transfer coefficient of short linear HPs, intended for creation the cooling systems of heat-stressed designs of spacecraft is carried out by making the HPs vapour channel forms of the Laval-liked nozzle. Comparison of the heat transfer coefficients of short HPs with the standard cylindrical vapour channel and the channel, made in the Laval nozzle form with the equality of all dimensions, flat evaporator shows that the HPs with the Laval-liked nozzle vapour channel exceeds the heat transfer characteristics of the standard HPs with a cylindrical vapour channel under high thermal loads. The study of the flow and condensation in such shaped vapour channels of the short HPs at high thermal loads gives an opportunity to analyze in detail the advantages of using such HPs. Capacitive sensors are additionally installed in cooled top covers of the HPs, and electromagnetic pulses with a frequency of 100 kHz were supplied to them from the external generator. At heating the HPs evaporator, starting from a certain thermal power threshold value, electromagnetic pulses became modulated. It is related with the formations of the boiling process in the capillary-porous evaporator and large amount of vapour over it and its discontinuous distribution. An analytical and numerical evaluation are applied to study the duration of the occurring pulsations, and the analytical results are compared with numerical and experimentally obtained values of the pulsations periods.

Summary regularities in heat transfer swirling flow within the tubular channels of power plants heat transfer devices

Heat transfer devices of «gas» – «gas» type are widely used for recovery of exhaust gases heat of power plants of different purposes, thereby improving their efficiency. In case of increase of heat flows, an increase in weight and size can be observed in such heat transfer devices, which is not always acceptable. An effective method to reduce them is to intensify the heat transfer. The issue of heat transfer intensification in these devices is associated with comparability of heat transfer coefficients on both sides of the heat transfer area. One of the effective ways of heat transfer intensification is to swirl the flow. Today swirling flows are successfully used in the elements of gas turbine equipment for the intensification of heat transfer processes. However, the represented in the literature empirical dependencies for calculating the heat transfer of swirling flows are private and require generalization. On the basis of the theoretical solution, there has been performed the generalization of experimental data on heat transfer and hydrodynamic resistance for axial-blade, auger-type and tangential ways to swirl the flow inside the tubular channels of heat transfer devices. The obtained function which takes into account the effect of relationship between circumferential and axial components of shear stress on the heat transfer allows us to trace the change of flow swirling along the channel. The difference between the obtained dependences and the experimental data did not exceed 9%.

The influence of different factors on the shape of pressure pulse at the liquid-vapor contact

The process of formation and propagation of the depression wave at spontaneous contact of cold liquid and saturated vapor is investigated in a gas dynamics approach. Modeling of wave processes was performed using Godunov’s method based on solving the Riemann problem on arbitrary discontinuity decomposition. The influence of various factors, namely, the kinetics of condensation and intensification of heat-transfer processes, on the pressure pulse form is investigated. Results of the investigation are in good agreement with results of modeling based on numerical solving the Boltzmann kinetic equation.

Intensification of heat transfer process: Improvement of shell-and-tube heat exchanger performances by means of ultrasound

Heat transfer in the presence of a low-frequency ultrasonic field has been investigated. Experiments were performed using a home-made shell-and-tube heat exchanger. The aim of this study was to investigate the effect of ultrasound on heat exchange performed by this new type of “vibrating” heat exchanger named sonoexchanger. Comparison was then made between overall heat transfer coefficients with and without ultrasound for the same hydrodynamic configurations. It was shown that under ultrasonic conditions, the overall heat transfer coefficient can be increased from 123 to 257%.

Intensification of heat-transfer processes in disperse system flow with solid phase deposition

The influence of the thickness of particle deposits and surface roughness on the heat-transfer and resistance coefficients is examined. It is shown that for high degrees of roughness resulting from the deposition of coarse particles, the efficiency decreases with increase in the thickness of the layer deposited, even at large Re numbers.