Heat Transfer Resistance Research Articles

Characteristics of heat transfer and flow resistance of magnetic fluid flow through porous media combined with magnetic field effect

The heat transfer and flow resistance of magnetic fluid flow under a magnetic field in the absence and presence of porous media were investigated experimentally in a rectangular duct. A ceramic foam, with a pore density of 13PPI (pores per inch), was used as a porous media. The mainstream flow rate varied between 0.1 and 3.7 l/min, while the magnetic intensity was adjusted to 0, 100, 300, and 500 mT. The results showed heat transfer was enhanced in both the non-porous and porous ceramic by applying the magnetic field to the magnetic fluid flow. Under a magnetic intensity of 500 mT, the heat transfer increments in the absence and presence of the foam were approximately 16.8% and 15.3%, respectively. By applying a magnetic fluid flow through the foam, the maximum heat transfer increment reached 171.7% compared to the absence of foam at the same flow rate. Notably, combining the use of a magnetic field and the ceramic foam resulted in a synergetic enhancement in heat transfer at low flow rates. The magnetic fluid flowing through the foam had a substantially higher flow resistance than the non-foam magnetic fluid flow. The pressure drop increased by applying a magnetic field to the magnetic fluid in the absence and presence of foam. The performance of the combined heat transfer techniques was evaluated in this research based on a performance evaluation criterion value. The results revealed that by applying the magnetic field, the performance evaluation criterion value of the magnetic fluid flow in the presence of foam was higher than in the absence of foam. Finally, the predictive formulas for Nusselt number and performance evaluation criterion as a function of Magnetic fluid heat transfer number were proposed.

Boosting Evaporative Cooling Performance with Microporous Aerogel.

Hydrogel-based evaporative cooling with a low carbon footprint is regarded as a promising technology for thermal regulation. Yet, the efficiency of hydrogel regeneration at night generally mismatches with vapor evaporation during the day, resulting in a limited cooling time span, especially in arid regions. In this work, we propose an efficient approach to improve hydrogel cooling performance, especially the cooling time span, with a bilayer structure, which comprises a bottom hydrogel layer and an upper aerogel layer. The microporous aerogel layer can reduce the saturation vapor density at the hydrogel surface by employing daytime radiative cooling, together with increased convective heat transfer resistance by thermal insulation, thus boosting the duration of evaporative cooling. Specifically, the microstructure of porous aerogel for efficient radiative cooling and vapor transfer is synergistically optimized with a cooling performance model. Results reveal that the proposed structure with a 2-mm-thick SiO2 aerogel can reduce the temperature by 1.4 °C, meanwhile extending the evaporative cooling time span by 11 times compared to a single hydrogel layer.

THERMAL STUDY OF COOLANT CIRCUITS OF BUILT-IN DEPHLEGMATORS OF DIABATIC DISTILLATION COLUMN

Diabatic distillation by using additional heat exchangers in the stages can significantly increase the energy efficiency of the column compared to adiabatic distillation. The paper proposes a calculation method for the condensation of ascending vapors on the tube surface of dephlegmators that are internally integrated on the trays of a multistage distillation column. The influence of parallel and series circuits of the coolant water supply to the dephlegmators on the technological parameters of the diabatic distillation unit is evaluated. For three types of coolant circuits, the following parameters are determined: thermal power, coolant temperature at the outlet of the tube dephlegmator, wall temperature on the condensate side, convective heat transfer coefficients, overall heat transfer coefficients, and thermal resistances on each tray of the distillation column. The results show that the parallel circuit to supply coolant water to the internal dephlegmators is the most suitable, providing the control of water flow rate on each tray and obtaining of the required temperature profile for height of the column to achieve the maximum efficiency of the separation process. For the parallel circuit, the heat transfer resistance can be concentrated in the heat transfer from both the coolant water to the wall of the dephlegmator tubes and the liquid condensed on the outer surface of these tubes.

Heat Transfer Characteristics of a Combination of Two Double-chamber Windows

Low-emissivity coatings on glass surfaces, inert gas instead of air between glasses, and double-glazed windows are usually used to reduce heat loss from a room through windows. This effect can also be achieved by installing two double-chamber windows in one window opening at a certain distance from each other. This work uses numerical modelling to study the characteristics of heat transfer through a system of two two-chamber windows. The distributions of temperature and airflow velocity due to natural convection in the chambers of double-chamber windows and the space between the windows are analyzed. The distributions of temperatures and heat fluxes over the surfaces of windows are determined depending on the distance between the windows. The dependences on the distance between the windows of radiation, convective, and total heat flow through a system of two double-chamber windows are studied. It is shown that two double-chamber windows located at a certain distance from each other make it possible to increase the heat transfer resistance compared to one two-chamber window without a low-emission coating by 2.6...2.8 times. As the distance between windows increases, a given system's overall heat transfer resistance increases. However, this increase becomes insignificant if this distance is greater than 80 mm.

Evaluation and comparison of thermal performance of latent heat storage units with shell-and-tube, rectangular, and cylindrical configurations

• Criteria are proposed to evaluate heat transfer, flow resistance, and irreversibility. • Shell-and-tube LHSU has the shortest phase change time and highest heat transfer rate. • Shell-and-tube LSHU achieves the highest j / f factor and the best thermal performance. • Cylindrical LHSU achieves the highest melting exergy efficiency of 81.5% • Shell-and-tube LHSU has the highest exergy efficiency of thermal store-release cycle. Latent heat storage plays a significant role in tackling the mismatch between energy supply and demand. Evaluation and comparison of the thermal performance of different shaped latent heat storage units (LHSU) are essential in the optimal design of LHSU. This study developed the numerical model of heat transfer and flow in shell-and-tube, rectangular, and cylindrical LHSU with the same PCM mass and heat transfer area. Two kinds of criteria, namely the j / f factor considering both heat transfer and flow resistance and the exergy coefficient evaluating irreversibility, are proposed to assess the performance of LHSU comprehensively. It shows that natural convection intensity in cylindrical LHSU is the highest, while the shell-and-tube LHSU exhibits the lowest phase change time and the highest heat transfer rate. The average j / f factor of shell-and-tube LHSU is 21.4% larger than that of rectangular LHSU and is twice as much as that of cylindrical LHSU. Therefore, the shell-and-tube LSHU achieves the best overall thermal performance. Moreover, cylindrical LHSU achieves the highest melting exergy efficiency of 81.5%, while shell-and-tube LHSU has the highest solidification exergy efficiency of 62.8%. Regarding the thermal storage and release cycle, the exergy efficiencies of shell-and-tube, rectangular, and cylindrical LHSU are 50.1%, 49.2%, and 48.5%, respectively. Therefore, shell-and-tube is the optimal shape to achieve the largest exergy efficiency of the thermal storage-release cycle. This study deepens the understanding of the thermal behavior of PCM packed in different shaped containers, obtains the optimal geometric configuration, and gives guidance to the optimal design of geometric parameters of the LHSU.

Numerical investigation of thermal hydraulic performance of an automobile heat transfer tube with ellipsoid dimples

The heat transfer tube is one of the key components affecting the heat transfer performance of automobile radiators. Proposes a new kind of heat transfer tube with ellipsoidal dimples based on the elliptical heat transfer tube. The effects of the arrangement and pitch ellipsoidal dimples on the turbulent heat transfer and flow resistance of the heat transfer tube are further studied by numerical simulation in the range of Reynolds number in 4080-24480. The results show that the ellipsoid dimple arrangement makes the near-wall fluid produce different flow forms, which enhances the turbulence degree of the tube fluid and thus improves the convective heat transfer capacity of the tube. Among them: vertical parallel arrangement (Case 1) causes convergent flow, oblique parallel arrangement (Case 2) causes helical flow, and diagonal symmetrical arrangement (Case 3) causes cross-helical flow. The cross-helical flow causes the most significant degree of turbulence, followed by the helical flow. Furthermore, the arrangement and pitch of the ellipsoidal dimples also have an essential influence on the heat transfer performance and flow resistance of the heat transfer tube. The Nusselt number and friction factor of Cases 1-3 increase successively. But the Nusselt number and friction factor gradually decrease with the rise of the pitch of ellipsoidal dimples. However, under different ellipsoidal dimples pitch, the comprehensive performance of Cases 1-3 is better than that of smooth elliptical tubes. Among them, Case 3 has the best performance when P = 15 mm, and the performance evaluation criteria value is up to 1.39.

Ensuring the energy efficiency of heat supply energy systems functioning by justifying the choice of glazing units for the external enclosing structures of buildings

The article is devoted to the further development and implementation of technologies that allow some considerable reducing energy consumption and, as a result, reduce greenhouse gas emissions. In Ukraine and abroad, a huge amount of heat is lost through the external enclosing structures of buildings. The largest specific heat losses occur through translucent enclosures, in particular, windows, the main elements of which are double-glazed windows. Today, manufacturers offer double-glazed windows, which are characterized by different costs and energy efficiency. The buyer does not know which double-glazed window to choose. We have proposed a clear and understandable method of solving this issue. For example, two different double-glazed windows were considered and a comparison was made on their cost, thermal resistance of heat transfer, and the ability to save certain amounts of heat and money for its purchase during operation. A feasibility study of additional capital investments in double-glazed windows with improved characteristics instead of conventional double-glazed ones has been carried out. Its results show that, at the current stage of the society development, the use of double-glazed windows with improved characteristics, despite their higher cost, is economically more appropriate than conventional double-glazed windows. This applies to external translucent fences of buildings of any purpose and form of ownership - both residential and industrial, public, which are on the balance sheets of local and state budgets, etc. The results of all calculations are explained in detail and presented in the form of a table. In addition, a simple and clear scheme of technical and economic substantiation of additional capital investments in a double-glazed window with improved characteristics is presented instead of an ordinary one, which can be used at the design stages of new construction or thermal renovation of existing buildings to ensure the selection of the most rational structural solutions. The materials of the article can be generalized to any other cases of technical and economic substantiation of the choice of options for technical solutions in energy-efficient projects.

Dependence of heat transfer resistance and air permeability resistance characteristics

The article is devoted to the analysis of the most popular designs of wall enclosure structures. Calculation of heat transfer resistance and air permeability of wall enclosure structures is carried out. The dependence of these characteristics is revealed. The most optimal design in terms of thermal characteristics and construction expenses were selected.

Verification of compliance with the requirements of the norms for the internal microclimate of a corner room

The article considers the buildings of the transport infrastructure that provides services to transport workers, e.g. medical and preventive, children's educational and educative institutions. The resistance to heat transfer of the windows was not increased for such buildings at the last change in the norms of thermal protection in the Russian Federation in 2018. In this regard, it is important to check compliance with the requirements of regulatory documents for radiation temperature and asymmetry of the radiation temperature at the boundary of the serviced area of the premises of such buildings. A literary review has shown that much attention is paid to these parameters in the world. Since the heat transfer resistance of the windows is much less than that of the outer walls, the influence of the outside air temperature on the temperature of the inner surface of the window is much greater than that of the walls. The proposed article considers a corner room of an intermediate floor with windows in the outer walls, occupying 35% of the area of the external enclosing structures, in Belgorod. The building is serviced by air heating. A comparison of the radiation temperature and the asymmetry of the radiation temperature with the same indicators in the ordinary room considered earlier showed that with windows of the specified size, the influence of the second window significantly reduces the radiation temperature and increases the values of the local asymmetry of the radiation temperature. At the same time, during the estimated winter period, not only optimal, but also permissible requirements of the regulatory document are not met at the border of the serviced area of the premises.

Modern studies of heat losses of a building basement through a soil mass

This study aims at considering the methods of calculating heat losses to the soil through the floor structures of the basement. Classical methods of calculating heat losses through the soil according to the updated set of rules are presented. Modern methods of calculation based on mathematical modeling have been studied. A full-scale experiment of one of the developed methods that proves the possibility of its application and the need for further enhancement has been considered. Methods for estimating the resistance to the heat transfer through the soil are introduced. The method of “concentric circles” is presented in comparison with the given values in Set of Rules 50.13330.2012. A comparative analysis of new methods with standard methods for calculating heat losses to the soil through the basement floor structures was carried out. Such methods as calculation “by zones” based on the principle of dividing the basement floor into strips have been studied. Various options for the building basement insulation that can affect the heat losses of the building have been considered. An analysis of the factors influencing heat loss such as the type of soil, the geometric dimensions of the building, the actual heat transfer resistance of the insulation, the temperature regime of the room and the presence of insulation has been presented. The formulas for calculating the complex heat-shielding characteristic in accordance with modern changes in the regulatory documentation are given.

Modelling and simulation of chemical reaction of porous MgCl2 pellets with NH3 by including impact of heat and mass transfer and structure change.

The MgCl2-NH3 reactive system is investigated in terms of heat and mass transfer coupled with chemical reaction through numerical simulation. The reversible nature of the chemical reaction is captured by including adsorption and desorption terms in the rate expression simultaneously. The kinetic coefficients of the adsorption are directly adopted from the literature, while those for the desorption reaction are calculated based on the thermodynamic relations. The impact of changing pressure and pellet porosity are also investigated in the simulations. The initial temperature of the pellet is 300 K in all simulations. Temperature, NH3 pressure, and conversion distributions in the pellets, along with pellet swelling are obtained and presented as a function of time. The results indicated strong effects of heat transfer resistances in the pellets.

Formation of local asymmetry of the resulting temperature at the boundary of the serviced area of the premises

The required resistance to heat transfer of windows has been normalized for a long time depending on the functional purpose of the building and the difference in the temperature of the indoor air and the temperature of the coldest five-day period. For residential and public buildings, they were in the range of 0.18 – 0.61 m²°C/W. In 1993, with the introduction of requirements for this indicator based on energy savings (according to the number of the degree-days of the heating period), they were increased for windows to the limits of 0.3 – 0.8 m²°C/W. From 15.06.2019, the Amendment №1 to SP 50.13330.2012 "Thermal protection of buildings" came into force and increased the required heat transfer resistances of residential and public buildings to the range of 0.49 –0.8 m²°C/W. Attention is drawn to the inadequacy of GOST 30494–2011 requirements as for measuring the local asymmetry of the resulting temperature in the center of the room. Such an erroneous requirement is that the radiation temperature, which is an integral part of the resulting one, depends on the measurement place. The radiation temperature in accordance with the V.N. Bogoslovsky's second comfort condition should be checked at the border of the serviced zone. Calculations of the local asymmetry of the radiation temperature, which has been measured at a distance of 0.5 m from the center of the window, showed that the optimal requirements to the center of the room are not met in none of the 19 cities of the Russian Federation for which the calculations were performed. Therefore, when forming the required resistance to the window heat transfer, it is proposed to take into account not only the number of the degree-days of the heating period, but also the calculated outdoor air temperature for the cold period of the year.

НОВЫЙ КОМПОЗИЦИОННЫЙ ДЕРЕВЯННЫЙ БРУС ДЛЯ МАЛОЭТАЖНОГО СТРОИТЕЛЬСТВА

The article discusses three main aspects of wooden low-rise housing construction that is environmental, economic and aesthetic ones. A wooden house is an environmentally friendly and healthy housing. Wooden houses usually cost less than stone or brick ones and require less energy for heating. Wooden houses can have a unique and at-tractive appearance thanks to various methods of finishing and decoration. The paper analyzes the new requirements for thermal engineering of enclosing structures and focuses on a new composite building material “Composite wooden beams” developed for low-rise housing construction. It presents the results of theoretical calculations and comparisons of heat transfer resistance for laminated timber and composite wooden beams, as well as the vapor permeability resis-tance of the new building material.

Study on topology optimization for performance improvement of heat transfer resistance

Study on topology optimization for performance improvement of heat transfer resistance

Energy-efficient construction materials in capital repair of apartment buildings (on the example of Rostov-on-Don)

This paper assesses the need to improve the energy efficiency of multi-apartment residential buildings in Rostov-on-Don, built in the period of industrial house building for the use of energy-efficient building materials. The paper analyses the existing housing stock, identifies the series of apartment buildings, determines the dynamics of construction by year, and the structural schemes of each series. Also, the heat transfer resistance of the building envelopes of all building series was calculated and high-performance insulation materials were selected according to the green materials assessment criteria.

Reducing the thickness of the insulation layer of building walls based on the study of their temperature and moisture regime

The thickness of the building insulation is calculated with the table values of the moisture of building materials, which is included in the regulatory documents with a margin. The study proposes the method for determining the thickness of the insulation layer using the values of operational moisture. To determine the operational moisture of materials, the equation based on the moisture potential is applied, the solution of which is sought using a discrete-continuum approach. The proposed equation makes it possible to determine the distribution of the moisture potential function in the wall enclosing structure. Thereafter, using the moisture potential scale, the distribution of mass moisture over the thickness of the enclosure for the period of maximum moisture accumulation is determined. The equation for determining the thickness of the insulation layer, in which the calculated value of the operational moisture is explicitly substituted, is derived. An algorithm for calculating the thickness of the insulation layer, taking into account the unsteady-state heat and moisture regime of the building envelope, is given. The application of the proposed method on a wall enclosing structure with insulation from expanded polystyrene boards is illustrated. It was found that the thickness of the insulation can be reduced by 26 mm from 120 mm to 94 mm while maintaining the reduced heat transfer resistance of the building wall. It is noted that the proposed method can be applied in the design of new wall enclosing structures.

Topological structures for microchannel heat sink applications – a review

The microchannel heat sink (MCHS) has the advantages of small heat transfer resistance, high heat transfer efficiency and small size, which exhibits good heat transfer performance in the field of active heat dissipation of electronic devices integrated with high heat flux density. In this paper, the application of MCHS in thermal management is reviewed in recent years, and the research progress of microchannel topology on enhancing heat transfer performance is summarized. Firstly, the research progress on the cross-sectional shape of the microchannel shows that the heat transfer area and fluid flow dead zone of the microchannel is the keys to affecting the heat transfer performance; Secondly, the microchannel distribution and the bionic microchannel structure have a great role in enhancing heat transfer performance, especially in microchannel temperature uniformity; Thirdly, the disturbing effect caused by interrupted structures in microchannels such as ribs and concave cavities has become a hot topic of research because it can weaken the thermal boundary layer and increase heat dissipation. Finally, the commonly used MCHS materials and cooling media are summarized and introduced. Based on the above reviews of MCHS research and applications, the future trends of MCHS topologies are presented.

The Numerical Simulation of Thermal Efficiency of Triple Pipe Heat Exchanger Using PCMs (Paraffin and Lauric Acid) System

COMSOL Multiphysics is used to adequately study the numerical modeling of phase change material behavior in a triple heat exchanger. The triple heat exchanger comprises three sides: an interior pipe for water cooling and heating, an annular space between the second and first interior pipes conveying Paraffin, and a third side annular space between the outer pipe and the second pipe holding Lauric acid. The dimensions of the triple heat exchanger are as follows: 30 cm length and side diameters of (0.5 in: 2.5 in: 3.5 in). Overall, the patterns reveal that the CFD and experimental work are in perfect agreement. PCM 2 is more reliable than paraffin. As the flow rate rises, the validity decreases; the maximum validation is obtained at 11 L/min. The results shows that the number of fins enhances efficiency by 5% (from 6 to 4); any number of fins over 6 promotes no more efficiency percentage. The additional fins are considered to produce increased heat transfer resistance, resulting in a significant increase in heat transmission. Copper slightly conducts heat better than aluminum and iron. Aluminum is the best material; its efficiency is equivalent to copper's. The diameter of the pipe has no effect on the percentage of paraffin efficiency. The better the heat resistance with an uniform heat transfer area between the paraffin and lauric acid layers, the broader the diameter of the lauric acid layer. Improving the outer diameter of the heat charge process implies increasing the efficiency percentage. The highest efficiency% is seen at 4.5 in.

EVALUATION OF THE NEW CONSTRUCTIVE-TECHNOLOGICAL SOLUTION OF THE FENCE STRUCTURE IN THE NON-REMOVABLE FORMWORK

The known constructive and technological solutions of enclosing constructions of buildings in fixed formwork with the use of qualitative, quantitative and general analysis are analyzed. Selected quantitative indicators, such as wall thickness, weight of 1 m2 of wall, heat transfer resistance, area of premises, cost, durability, tendency to shrinkage. Qualitative indicators were used: resistance to the sun, mold, fertilization and destruction, features of delivery of building materials. The method of multicriteria analysis is used to evaluate new structural and technological solutions of fencing structures and determine the most effective technologies. Categories of various constructive-technological decisions in a fixed timbering on the basis of economic consideration of modeling are considered. Such modern constructive-technological decisions of enclosing constructions in a fixed timbering are selected for comparison, as: expanded polystyrene blocks; blocks with wall alignment system; polystyrene concrete blocks; concrete blocks; blocks of cement-chip material; large panels made of expanded polystyrene; three-layer reinforced expanded polystyrene panels; cement particle boards; concrete slabs; frame-monolithic formwork with cement-chipboard cladding; frame-monolithic formwork with cladding of glass-magnesium plates and technology according to the patent UA 149402. The new design and technological solution, which has been developed and patented, is a combination of modern materials that are of great interest and demand to consumers, such as light steel thin-walled structures and expanded polystyrene concrete. The evaluation of technological solutions by quantitative criteria on a ten-point scale, where the minimum and maximum values are assigned points 1 and 10, respectively. Graphical modeling of the summary variants, which are equalized, is performed, and the corresponding diagrams are constructed.

Topology design of cold plates for pouch battery thermal management considering heat distribution characteristics

The topology optimization technique was applied to the structural design of the cold plate for pouch Lithium-ion batteries in this study, aiming to boost its overall performance. With maximum heat transfer and minimum flow resistance as the optimization objectives of the cold plate channel, we proposed a topology approach for non-uniform heat generation, motivated by the inhomogeneous cell heat distribution. The optimization iteration yielded a non-uniform channel structure (N-type) with a denser channel distribution in the hot zone. According to the topology results, a simplified cooling system model for the 50 V battery module was constructed and investigated numerically. Under constant and dynamic current conditions, the N-type cold plate was compared to cold plates with other two channel structures: the uniform channel type and the straight channel type. As a result, the N-type cold plate showed the best comprehensive performance. Furthermore, the optimal design of the cooling system with different inlet and outlet layouts, channel depths and inlet mass flow rates was explored. The N-type cold plate with bottom inlet-top outlet layout, channel depth of 2.5 mm, inlet width of 15 mm and mass flow rate of 8 g s-1 is the ideal choice for the cooling system at 2C discharge. The j/f factor reached the maximum value of 3.42, with the pump power consumption and the Nusselt number being 2.53×10-3W and 39.26, respectively. The topology cold plate and optimization method proposed in this study can be used to increase the performance of BTMS.