Heat Transfer Intensity Research Articles

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.

Design of effective heat transfer structures for performance maximization of a closed thermochemical energy storage reactor through topology optimization

This study addresses the need for heat transfer intensification in closed thermochemical energy storage reactors using topology optimization as a design approach. We introduce a novel topology optimization framework to simultaneously optimize fins geometry and amount of enhancer material while meeting specific discharge time, bed size, and bed porosity requirements. The proposed topology optimization framework is thoroughly tested by optimally designing innovative fin structures in a reference thermochemical storage reactor aimed at heat storage in industrial applications and operated with Strontium Bromide in the range 150-250 °C. The generated designs show performance improvement up to +286% compared to state-of-the-art designs. Our findings also indicate that the optimal amount of enhancer material varies significantly; large bed sizes with high packing factors maximize reactor energy density while highly packed reactive beds provide a larger amount of energy in fixed discharge times compared to less packed reactive beds. Finally, the benefits and limitations of the proposed topological optimization approach, as well as the extent to which the optimal designs found are generally applicable are thoroughly discussed to provide guidelines for configuring high-performing closed system thermochemical energy storage reactors.

Analysis of the Heat Exchanger Energy Efficiency of Variable Cross Section with an Inhomogeneous Coolant

Abstract One of the main priorities in the modern thermal power engineering development is the problem of energy saving due to the economical use of fuel and energy reserves. Increasing energy consumption with a simultaneous increase in energy prices and widespread environmental degradation necessitates the development and implementation of energy efficient technologies to save fuel, materials and labour costs. The object of study is tubular heat exchangers of variable cross section, which are widely used in steam generators of nuclear power plants, gas turbines and transport plants. Scale deposit properties and the composition of the heat coolant were studied, and their influence on the energy efficiency of heat exchangers was analysed. To study the scale deposits and coolant influence on the energy efficiency of heat exchangers, their properties were examined using an atomic emission analysis with the help of a TESCAN electron microscope. The principles of implementing technologies aimed at intensifying heat transfer, reducing hydraulic and heat losses in heat exchangers were formulated.

The Effect of Boundary Conditions on the Modeling of Anomalous Intensification of Turbulent Heat Transfer in an Inclined Groove in the Wall of a Narrow Channel

The Effect of Boundary Conditions on the Modeling of Anomalous Intensification of Turbulent Heat Transfer in an Inclined Groove in the Wall of a Narrow Channel

Zastosowanie programowania nieliniowego dla konfiguracji systemu wymiany ciepła w aparatach kolumnowych przemysłu petrochemicznego przy minimalnych kosztach

For increasing the efficiency of heat transfer processes, the layer-by-layer concept of arrangement of various types of packings in column apparatuses is considered. The results of hydraulic tests of a dry and irrigated cell packing, as well as individual hydraulic tests of a mesh packing, are presented. Methods and algorithms for solving linear programming problems under “uncertainty” conditions were considered. However, in some areas of science it is often difficult or impossible to formalize the problem in an appropriate way and reduce it to a linear programming problem. In this paper, methods for solving non-linear programming problems with a vector objective function are considered. At present, the use of non-linear programming in the vast majority of real situations is reduced to linear approximation models. Along with this, at a significant non-linearity, due to its specificity or influence on the nature of the model, it is necessary to apply optimization methods that are much more complex than, for example, the simplex method. However, the importance of non-linear programming is constantly increasing. This is due to the rapidly growing knowledge of managers and specialists in the use of mathematical models designed to prepare solutions, as well as the increasing availability of computer programs for solving large-scale nonlinear problems. The analysis and studies of the hydrodynamics of a number of regular packings have shown that cell and mesh packings are promising for the implementation of the phase inversion mode. The proposed concept of intensifying heat transfer processes in column apparatuses is based on the use of layers of various packings arranged in the following order in the apparatus, type 1 – cell, type 2 – mesh. At the same time, their main geometric characteristics differ significantly from each other. However, the structure of the bulk packing is inhomogeneous, which makes it difficult to implement a stable mode of local phase inversion under these conditions. This is due to the structure of column apparatuses with bulk packing, in which there is an increased proportion of pores (porosity) near the walls of the apparatus. Porosity is very significant; it can reach up to 40%, and as a result, the local velocity near the walls exceeds the velocity in the centre of the apparatus by up to 70%. By contrast, the use of regular packing structures makes this potentially highly efficient mode technically possible.

Experimental analysis of heat transfer characteristics using ultrasonic acoustic waves

In this experimental work, heat transfer intensification using ultrasonic waves was investigated. A heat source, consisting in a parallelepiped aluminum block in which two electrical heating cartridges of 160 W each were mounted to heat four liters of distilled water contained in a tank made of Plexiglas. To demonstrate the effectiveness of heat transfer enhancement with the use of ultrasounds, three different configurations were analyzed. In the first one, considered as a reference case, the heat transfer was studied without ultrasound field. In the second configuration, ultrasonic acoustic waves were generated using one transducer vibrating at a fixed frequency of 40 kHz with a total power of 60 W. In the last configuration, ultrasounds were generated with two similar transducers mounted on two opposite walls of the water tank while maintaining the same power and frequency. The effect of the distance separating the heat source to the trans-ducers on the convective heat transfer coefficients and the average temperature of the water in the tank were analyzed in detail. The results revealed that the natural convection heat transfer in the water tank was intensified by means of low frequency acoustic waves. Indeed, it was shown, particularly, that more the distance between the transducer and the heater is low more the heat transfer improvement is better. The heat transfer enhancement factor was estimated to 2.5 on the surface facing the transducer while it was only about 1.2 on the opposite surface in C2 configura-tion. In C3 configuration, the heat transfer enhancement factor is nearly the same with, however, more homogenous water temperature. The acoustic cavitation and streaming were identified as the main phenomena leading to these results. This study successfully demonstrated the feasibility of heat transfer intensification using low frequency ultrasonic waves.

C conjugate heat transfer simulation of swirl internal cooling on blade leading edge

Abstract Swirl cooling can not only increase the area of the heat exchange wall covered by the coolant, but also improve the average heat transfer intensity and uniformity of the target surface. SST k-ω turbulence model is utilized in the conjugate heat transfer numerical simulation. Based on C3X blades, leading edge swirl cooling structure of the corresponding areas are modified. The flow and heat transfer characteristics of swirl cooling are analyzed at different cross-sections and positions. It is found that there exists an optimal aspect ratio and hole spacing to minimize the temperature gradient on the swirl cavity wall. The swirling motion in the swirl cavity can significantly increase the heat transfer coefficient of the wall surface.

Numerical Simulations on Combustion and Heat Transfer in Porous Medium with Different Gyroid Triply Periodic Minimal Surfaces Structures

ABSTRACT To investigate the influence of the porous medium with Triply Periodic Minimal Surfaces (TPMS) structures on lower concentration methane combustion. This study analyzed the flow field and heat transfer in the porous medium with three types, i.e. an axial gradual period type, a conical type and a uniform type, through the pore-scale simulation (PSS) approach. In cases of this study, the flashback equivalence ratio of the conical PMB is about 12.5% lower than that of the gradient PMB at the same inlet flow velocity, and the blow-off inlet flow velocity of the conical PMB is about 30% higher than that of the gradient PMB at the same equivalence ratio. Moreover, the flame shape is closely related to the local solid structure and flow velocity. The solid surfaces radiation heat transfer occurred mainly in regions with high solid temperature gradients. Additionally, the absolute value of the radiant heat flux is about 1/4 to 1/5 of the total heat flux, which reflects the proportion of thermal convection in the heat transfer on the solid surface. Convective heat transfer between solid and fluid occurred mainly in the areas of heat recuperation known as preheat zone, and higher inlet flow rates significantly increase the upstream convective heat transfer intensity.

Heat transfer characteristics and cooling capacity of online quenching equipment for plate

The finite element method was taken to simulate the heat transfer characteristics and capacity of the online cooling equipment of a 3500 mm medium thickness plate production line. The cooling field on the steel plate surface could be divided into the impact zone and the wall jet zone after cooling water was sprayed on the surface of the steel plate. The impact zone had an intense heat transfer capacity because of the static pressure and the high radial velocity of cooling water. The maximum heat transfer coefficient (HTC) in the impact zone of spray and laminar cooling equipment could reach 20,334 and 10,096 W/(m2 °C), respectively. In the wall jet zone, the radial velocity of cooling water was the main factor affecting the heat transfer capacity. With the decrease in velocity, the HTC decreased gradually. There is a linear relationship between the average HTC and the water flow. The average HTCs of spray and laminar cooling equipment in the common range of water flow were 5904–7255 W/(m2 °C) and 3012–3369 W/(m2 °C), respectively. The HTCs were verified in the 3500 mm medium thickness plate production line.

Nanofluids in microchannel heat sinks for efficient flow cooling of power electronic devices

Nanofluids hold significant potential in the field of heat transfer intensification. The objective of this paper is to investigate the thermal-hydraulic characteristics of Ag, SiO2, and Al2O3 nanofluids in rectangular (MC-R), trapezoidal (MC-T), and omega-shaped (MC-O) microchannel heat sinks (MCHSs) through conjugate numerical simulations under a constant heat flux of 100 W/cm2. Water was used as the base fluid, and nanoparticle volume fraction ranged from 0 to 6 %. The average heat transfer coefficient (HTC), Nusselt number, thermal resistance, pressure drop, and pump power as a function of Reynolds number are studied for different nanofluids and cross-sectional shapes in microchannels. The overall performance is also characterized by thermal performance factor, coefficient of performance (COP), and entropy generation analysis. The simulation results show that adding Al2O3 nanoparticles with a volume fraction of 6 % in MC-R at a Reynolds number of 500 results in a substantial 12.3 % increase in the average HTC and an approximately 8 % reduction in total thermal resistance compared to pure water. For the same nanofluid, MC-O exhibits the highest convective HTC, the highest Nusselt number, and the lowest maximum heat sink temperature. The total entropy generation in the MCHSs decreases obviously with increasing Reynolds number. MC-O has the smallest total entropy generation, meaning it has the optimal flow and heat transfer irreversibility. These findings can not only provide valuable guidance for the rational design of MCHSs but also hold significant implications for the effective cooling of microelectronic components via nanofluid-based thermal management technology.

Evaporation and heat exchange of a thin liquid layer under various heating methods: Advantages of local heating over uniform heating of the wall

Intensification of heat transfer and evaporation is widely used in modern technologies at limited dimensions of heat exchange surfaces and high energy density. To date, stationary heat transfer at uniform wall heating, has been most thoroughly investigated. Meanwhile, there is no comprehensive research providing a comparison of free convection, heat transfer and evaporation with different heating methods. The novelty of the work lies in the comparison of the heat transfer efficiency using one laser beam, two beams and homogeneous heating. When heated by a single laser beam, the average temperature of the layer interface is significantly lower than at homogeneous heating. However, due to the generation of intense Marangoni flow, a small heating spot provides more intense evaporation than homogeneous heating. The effectiveness of heat transfer is analyzed in terms of the total efficiency coefficient. If it is important to take into account power input, it is preferable to use a single laser beam. If it is not decisive, then two laser beams allow achieving maximum heat transfer and evaporation rate. Homogeneous heating is significantly inferior to local non-stationary heating in terms of all key criteria. The simple expressions serve to optimize the total efficiency coefficient.

The Effect of Boundary Conditions on the Modeling of Anomalous Intensification of Turbulent Heat Transfer in an Inclined Groove in the Wall of a Narrow Channel

The effect of the boundary conditions of the first and second kind on the predicted characteristics of anomalous heat transfer intensification in the case of turbulent low-velocity air flow over an inclined groove in the hot wall of a narrow channel is numerically investigated. It is shown that the isothermality conditions lead to a considerable underestimation of the thermal efficiency of a surface structured with narrow grooves, compared with the conditions of heat flux constancy.

Study on Phase Change Flow and Heat Transfer Characteristics of Microalgae Slurry in the Absorber Tube of a Parabolic trough Solar Collector

The study of gas–liquid two-phase flow and heat transfer in non-Newtonian fluids is of great significance for the research and development of refrigeration and energy storage. In this paper, the characteristics and influencing factors of the phase change reaction in microalgae slurry were studied by numerical simulation and experimental verification. In order to further study the rheological and heat transfer characteristics of gas–liquid two-phase flow in the collector, the effects of wall heat flux, inlet velocity and microalgae slurry concentration on the phase change reaction in microalgae slurry were studied. The results show that when the boundary conditions of microalgae slurry with the same concentration change, the phase transition of microalgae slurry is different. The higher the wall heat flux, the more forward the phase transition occurs, and the smaller the flow rate, the more forward the phase transition occurs. When the boundary conditions remain unchanged, the phase transition point of microalgae slurry with different concentrations is the same, and the concentration of microalgae slurry will not be affected. However, the deviation between the fluid temperature and the thermal conductivity of high-concentration fluid after phase change is larger than that of low-concentration fluids. The deviation in the fluid temperature reaches approximately 10 K, and the deviation in thermal conductivity reaches approximately 0.025 W/(m·K). Therefore, the change in the fluid temperature and heat transfer intensity after phase change in microalgae slurry is more intense than that of Newtonian fluids.

Evaporation of ferrofluid drop in magnetic field in Leidenfrost mode

When a droplet evaporates in the Leidenfrost mode, the intensity of heat transfer is significantly reduced compared to the boiling mode, which is highly undesirable for a number of applications. Here we demonstrate a method for intensifying heat transfer during droplet evaporation in the Leidenfrost mode. The essence of this method is that if a magnetic nanofluid is used as a cooling liquid, then with the help of an inhomogeneous magnetic field it is possible to forcibly press a levitating drop to a superheated surface, which contributes to the intensification of the drop evaporation. An aqueous suspension of Fe3O4 is used here as a ferrofluid. The experimental results show that this method can reduce the drop evaporation time by several times. When the magnetic field gradient increases, the drop evaporation time decreases according to the power law. A simple phenomenological model is proposed to substantiate the observed effects.

Numerical Modeling and Simulation of Rooftop Thermal Photovoltaic Chimney for Buildings' Electrical Energy Generation and Passive Cooling

The present study consisted mainly of making the thermal and dynamic analysis of the fluid flows in mixed convection in the chimney integrated into the building with an upper-covered horizontal hybrid thermal photovoltaic collector on a slab roof. The governing equations discretized by the finite difference method resulted in the systems of tri-diagonal algebraic equations, which were solved by Thomas' algorithm and Gauss Seidel's iterative method. Numerical solutions are presented for various geometrical aspect ratios, Rayleigh, and Reynolds numbers. The results are presented in terms of streamlines, isotherms, velocity, heat transfer intensity, and PV cells’ electrical efficiency versus the governing control parameters in detail.

CONVECTION OF A VISCOUS INCOMPRESSIBLE COOLANT IN A HORIZONTAL CYLINDRICAL PIPE HEATED FROM BELOW. AN ANALYTICAL SOLUTION

To intensify heat transfer processes in nuclear power, it is proposed to use natural convection in horizontal heat exchange tubes. In the article, the problem of convection of a viscous, incompressible fluid in a horizontal cylinder heated from below is analyzed analytically. The proposed analytical method is based upon the use of basic functions that satisfy the initial equation but do not satisfy the boundary conditions. The problem with boundary conditions with a sufficient degree of accuracy is solved by dividing the area of the square described around the cylinder into a large number of smaller element cells. This separation has made it possible to obtain analytical expressions of critical Rayleigh numbers for different positions of current lines in a horizontal cylinder with a viscous, incompressible fluid.

Heat Transfer Analysis in Sound-Enhanced Fluidized Beds Containing Geldart B-C Binary Mixtures

An experimental study was carried out on the binary mixing behaviour of Group C with Group B particles as well as the heat transfer behaviour between the bed and submerged heating tube in an acoustic field. The effects of excessive air velocity, sound pressure intensity, and local and averaged heat transfer coefficient were studied for five binary mixes. Different binary mixture fluidization ratios with and without sound aid are used to compare the results. The study also discusses the measurement techniques used, including pressure measurements and the characteristics of the bed solids. Sand and hydrophilic bituminous coal are used as the binary mixture materials, with different proportions of each. The experiments are conducted both with and without acoustic assistance to compare the outcomes. This study provides insights into the heat transfer behavior and binary mixing in a sound-assisted fluidized bed. The findings indicate that sound assistance has a significant impact on the heat transfer performance of Geldart B-C binary mixtures in fluidized beds. It highlights the potential benefits of using acoustic energy to improve fluidization quality in binary systems.

Analysis of the thermal improvement of plate fin-tube heat exchanger with straight and curved rectangular winglet vortex generators

A novel combination of curved and straight rectangular winglet vortex generator (RWVG) pairs was arranged in a common-flow-up (CFU) configuration on plate fins for enhancing the comprehensive performance of the fin-tube heat exchanger. The effects of RWVGs were compared with those of non-RWVGs for flow streamline, temperature distribution, and pressure field. Then the thermohydraulic performance and the comprehensive performance were numerically investigated among the cases with varied locations and sizes of RWVGs. The results revealed that j for all cases with RWVGs was higher than non-RWVGs and the values of JF were greater than 1.0 for all cases except for case 4. Especially, apparent improvements in heat transfer characteristics and comprehensive performance were observed with the novel combination. Furthermore, it was found that the appropriate location and size of RWVGs were able to influence the thermohydraulic characteristic positively. The curved RWVGs with a smaller offset showed better comprehensive performance, and the larger attack angle of the straight RWVGs was accompanied by a higher heat transfer intensity and a greater flowing drag force. Moreover, the curved RWVGs with a larger arc angle performed better in the comprehensive performance, and the length of the straight RWVGs was larger and JF smaller.

Radiative–Convective Heating of the Surface of the Martian Descent Vehicle MSL with Taking into Account the Turbulent Nature of Flow

The three-dimensional computer model based on the Reynolds-averaged Navier–Stokes equations together with the Baldwin–Lomax and Prandtl algebraic models of turbulent mixing is used to calculate radiative–convective heat transfer on the surface of the MSL descent space vehicle. The intensification of convective heat transfer on the leeward side of the frontal aerodynamic shield and the superiority of the radiative heat flux density over the convective one on the rear surface are demonstrated. The calculations are performed using the physically and chemically nonequilibrium gas model. The results are compared with the calculations based on other computational models and the flight data on the heat load to the descent vehicle obtained during MSL descent in the dense layers of the Martian atmosphere.

Tests for Validation Problems of Anomalous Intensification of Separation Flow and Heat Transfer on Structured Surfaces with Extraordinary Pressure Differences

Tests for Validation Problems of Anomalous Intensification of Separation Flow and Heat Transfer on Structured Surfaces with Extraordinary Pressure Differences