Heat Exchanger Surfaces Research Articles

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.

Improvement of the vacuum evaporator for the production of paste-like semi-finished products with a high degree of readiness

The object of this study is the process of cooking blended pureed mass based on apple, Jerusalem artichoke, cranberry, and hawthorn in an improved vacuum evaporation apparatus with a unified stirrer for the production of paste-like semi-finished products with a high degree of readiness. Conventional devices for cooking are associated with intermediate heat carriers, significant energy and metal consumption, the difficulty of stabilizing the temperature field under the conditions of a significant duration of the process, which prevents ensuring mobility, resource efficiency, and reducing the quality of resulting products. This led to the improvement of the vacuum evaporator by replacing the steam jacket with a film-like resistive electric heater of the radiating type, with the simultaneous use of a unified stirrer to increase the useful heat exchange surface by 0.6 m2. This was achieved by heating the internal space of the stirrer with the heater mentioned above, thereby forming the total area of heat exchange up to 2.8 m2, which is 28 % more than the area of the prototype (2.2 m)2. The rheological parameters of the resulting fruit and vegetable paste at a boiling temperature of 50...55 °C at a shear rate of 0.8...2.0 s-1 were determined, while the effective viscosity was in the range of 5.0...18.0 Pa·s. At the same time, the character of boiling kinetics in the improved vacuum-evaporating apparatus was 30 % lower than the indicator for the basic design MZ-2S-241a. The improved vacuum-evaporator with a unified stirrer is characterized by reduced specific heat consumption for heating the loaded pureed semi-finished product by 13.8 %. By increasing the heating surface by 28 % and reducing the duration of the temperature load on the product according to experimental data in the device by 29.6 %

Mixed Convective Heat Transfer Characteristics of Graphene Nanofluid Strengthened by Periodically Direction-Switching Electric Field

Abstract Transformer-oil with low thermal conductivity and large viscosity has poor heat dissipation capability, which leads to the thermal drive failure caused by transient overload. To improve its cooling capability, this paper has proposed first the method which combined the periodically direction-switching electric field and graphene nanofluid to enhance the mixed convective heat transfer properties of transformer-oil, and analyzed the effects of switching periods, nanofluid concentration, electric field strength, heat flux, and Reynolds number on mixed convection heat transfer experimentally. The results show that the heat transfer characteristic of transformer-oil is improved up to 52% by the periodically direction-switching electric field and graphene nanofluid. As the switching period decreases, the thermal performance of the suspension is enhanced more significantly. Moreover, by analyzing the heat transfer mechanism, the periodically direction-switching electric field causes the nanoparticles to move reciprocally, repeatedly impacting and breaking the boundary layer of the heat exchange surface to enhance the perturbation, thus enhancing the heat transfer effect. Meanwhile, the predicted correlation has been proposed on the basis of influence factors, which are in good agreement with the experimental data.

Optimization mathematical model of a contact air cooler for a mine turbocompressor

Purpose. Establishing the dependencies of rational parameters of turbocompressor contact air coolers on the operating mode and initial conditions. Methodology. The methods of analytical research, mathematical modeling, physical modeling, and mathematical statistics were used in the study. Findings. As a result of the research, an optimization mathematical model of the mine turbocompressor air cooler was developed, which allows establishing its rational parameters depending on the initial conditions and operating modes of the turbocompressor. The adequacy of the theoretical studies was proved on a specially designed experimental setup. The obtained dependences make it possible to minimize the theoretical flow rate at the inlet to the uncooled section of the turbocompressor, which reduces the specific energy consumption for the compressed air production. Originality. For the the first time, a method has been developed for determining the rational parameters of contact air coolers when the initial temperature of water, air, and air pressure changes, which allows developing a methodology for the constructive calculation of the contact cooling system of mine turbocompressors. Practical value. Compressed air is widely used in all industries. It is one of the most common energy carriers in industrial enterprises, and the devices associated with its distribution and processing are an energy-intensive complex industrial energy system; the level of its perfection depends on the performance of technological processes that use compressed air. Compressed air is widely used in the mining industry (ore mining and fuel production). Compressed air is produced by turbocompressors. To increase the efficiency of the compressor, compressed air coolers are used. A significant weakness of the standard compressor cooling system is the rapid contamination of the heat exchange surfaces of air coolers with scale layers, which leads to a decrease in their efficiency and an increase in the specific energy consumption for compressed air production. This disadvantage is not found in the Venturi tube – centered droplet separator contact air coolers. As a result of the study, the dependencies were obtained and used to develop a methodology for the constructive calculation of contact air coolers for a mine turbocompressor.

Long-term operation optimization of circulating cooling water systems under fouling conditions

Fouling caused by excess metal ions in hard water can negatively impact the performance of the circulating water system (CCWS) by depositing ions on the heat exchanger's surface. Currently, the operation optimization of CCWS often prioritizes short-term flow velocity optimization for minimizing power consumption, without considering fouling. However, low flow velocity promotes fouling. Therefore, it’s crucial to balance fouling and energy/water conservation for optimal CCWS long-term operation. This study proposes a mixed-integer nonlinear programming (MINLP) model to achieve this goal. The model considers fouling in the pipeline, dynamic concentration cycle, and variable frequency drive to optimize the synergy between heat transfer, pressure drop, and fouling. By optimizing the concentration cycle of the CCWS, water conservation and fouling control can be achieved. The model can obtain the optimal operating parameters for different operation intervals, including the number of pumps, frequency, and valve local resistance coefficient. Sensitivity experiments on cycle and environmental temperature reveal that as the cycle increases, the marginal benefits of energy/water conservation decrease. In periods with minimal impact on fouling rate, energy/water conservation can be achieved by increasing the cycle while maintaining a low fouling rate. Overall, the proposed model has significant energy/water saving effects and can comprehensively optimize the CCWS through its incorporation of fouling and cycle optimization.

Prediction model of CaCO3 crystal-particle mixed fouling on heat transfer surface

Predicting the fouling of heat exchange surface without affecting the production order is crucial. Conventional methods for predicting the fouling, e.g., numerical model and simulating calculation, are not applicable under complex environment with mixed fouling. To address this vital problem, in this article, we present a prediction model for calculating crystallization - particle mixed fouling deposition on heat transfer surfaces based on Kern-Seaton model theory, which considers the process of deposition and removal during fouling generation. A calculation formula is derived to determine the fouling resistance of crystalline - particle mixed fouling, which aims to predict the fouling condition on heat transfer surface according to environmental parameters. To confirm that the suggested method works, an experimental rig is built. It is found that the proposed method calculates the fouling resistance variation curve of time. An experimental results show that the proposed fouling prediction model can accurately predict the fouling resistance on the heat exchange surface, which average relative error is 7.97% compared with the experimental data.

DAMAGE TO THE HEAT EXCHANGE SURFACE OF STEAM GENERATOR PGV-1000

A comprehensive analysis of the damaged heat exchange surface of the PGV-1000 steam generator during long-term operation, their relationship with hydrodynamic, water-chemical regimes and surface properties was conducted. The methods of minimization of damages during the operation of Ukraine's NPP are considered.

Retardation of CaCO3 fouling on heat exchanger surface using water-based cloves-functionalized multiwall carbon nanotubes (C-MWCNT) nanofluids

Retardation of CaCO3 fouling on heat exchanger surface using water-based cloves-functionalized multiwall carbon nanotubes (C-MWCNT) nanofluids

Numerical Investigation on the Performance of Horizontal Helical-Coil-Type Backfill Heat Exchangers with Different Configurations in Mine Stopes

The application of ground heat exchanger technology in backfill mines can actualize subterranean heat storage, which is one of the most effective solutions for addressing solar energy faults such as intermittence and fluctuation. This paper provides a 3D unsteady heat transfer numerical model for full-size horizontal backfill heat exchangers (BFHEs) with five configurations in a mining layer of a metal mine by using a COMSOL environment. In order to ensure the fairness of the comparative analysis, the pipes of BFHEs studied have the same heat exchange surface area. By comparing and evaluating the heat storage/release characteristics of BFHEs in continuous operation for three years, it was discovered that the helical pipe with serpentine layout may effectively enhance the performance of BFHEs. Compared with the traditional SS BFHEs, the heat storage capacity of the S-FH type is significantly increased by 21.7%, followed by the SA-FH type, which is increased by 11.1%, while the performances of U-DH and SH type are considerably lowered. Also, the impact of the critical structural factors (pitch length and pitch diameter) was further studied using the normalized parameters C1 and C2 based on the inner diameter of the pipe. It is discovered that BFHEs should be distributed in a pipe with a lower C1, and increasing C2 encourages BFHEs to increase the storaged/released heat of BFHEs. By comparatively analysing the effect of thermal conductivity, it is found that the positive effects of thermal conductivity on the performance of SH, U-DH, SA-FH, and S-FH type BFHEs are found to decrease successively. This work proposes a strategy for improving the heat storage and release potential of BFHEs in terms of optimal pipe arrangement.

Wetting Behavior of CO2 Droplets on Smooth Solid Surface: Molecular Simulation Perspective

The wettability of droplets on solid surfaces is important for accurately revealing the microscopic mechanisms of gas condensation nucleation and droplet growth. During the contact condensation of CO2 gas on the heat exchanger surface in the pressurized liquefied natural gas technology, the wettability of CO2 droplets on the heat exchanger surface directly affects the heat transfer thermal resistance of the heat exchanger, which then affects the heat transfer efficiency of methane and ethane in the heat exchanger. Therefore, molecular dynamics simulations were used to study the spreading process and wetting patterns of nanoscale CO2 droplets on different energy surfaces. The results show that as the potential well depth ε of the wall atoms increases, the intensity of the solid-liquid interaction increases and the corresponding surface energy increases accordingly, showing different droplet spreading rates and wetting characteristics. Unlike the interfacial characteristics of macroscopic droplets, there are significant fluctuations at the gas-liquid interface of droplets on the molecular scale, but microdroplets can still form a specific contact angle after spreading on different energy surfaces in a statistical sense, and this contact angle decreases with increasing intensity of solid-liquid interaction. The low-energy surface at potential well depths ε less than 266 J·mol-1 exhibits a CO2-phobicity, and the surface becomes CO2-philic as the potential well depth continues to increase. The trend of the contact angle of CO2 droplets affected by temperature is the same as that of the center-of-mass height, which characterizes the spreading morphology of CO2 droplets. As the temperature increases, the contact angle decreases due to the further spreading and wetting of droplets on different energy surfaces. As the CO2-philicity of the surface gets higher, the contact angle decreases to a greater extent.

Enhancing the Fuel Efficiency of Cogeneration Plants by Fuel Oil Afterburning in Exhaust Gas before Boilers

Cogeneration or combined heat and power (CHP) has found wide application in various industries because it very effectively meets the growing demand for electricity, steam, hot water, and also has a number of operational, environmental, economic advantages over traditional electrical and thermal systems. Experimental and theoretical investigations of the afterburning of fuel oil in the combustion engine exhaust gas at the boiler inlet were carried out in order to enhance the efficiency of cogeneration power plants; this was achieved by increasing the boiler steam capacity, resulting in reduced production of waste heat and exhaust emissions. The afterburning of fuel oil in the exhaust gas of diesel engines is possible due to a high the excess air ratio (three to four). Based on the experimental data of the low-temperature corrosion of the gas boiler condensing heat exchange surfaces, the admissible values of corrosion rate and the lowest exhaust gas temperature which provide deep exhaust gas heat utilization and high efficiency of the exhaust gas boiler were obtained. The use of WFE and afterburning fuel oil provides an increase in efficiency and power of the CPPs based on diesel engines of up to 5% due to a decrease in the exhaust gas temperature at the outlet of the EGB from 150 °C to 90 °C and waste heat, accordingly. The application of efficient environmentally friendly exhaust gas boilers with low-temperature condensing surfaces can be considered a new and prosperous trend in diesel engine exhaust gas heat utilization through the afterburning of fuel oil and in CPPs as a whole.

A review of the use of scraped surface heat exchangers to crystallize fats and fat‐based products

AbstractScraped surface heat exchangers (SSHE) are very important in many industries including chemical, cosmetic, pharmaceutical, and food. Some of the applications of SSHE in the food industry are to control the crystallization process in fats and frozen desserts, among other applications. This can be achieved because of the scraping action of the fixed blades that continuously rotate within the cooling surface removing the crystallized product from the surface, improving heat transfer, preventing crystal build‐up on the wall, and accelerating the crystallization process. There are many process parameters that can be controlled in the SSHE to control and tailor the fat crystallization process, and many fat‐based products can benefit from this machine, such as shortenings, spreads, and chocolates. Moreover, SSHE have recently been coupled to other processing devices such as high intensity ultrasound (HIU), high pressure homogenizers, and others to improve the crystallization process even further. This review focuses on the importance of SSHE in fat and fat‐based products and discusses the importance of the parameters of the SSHE in the crystallization process, as well as how they affect the physical properties of different fat sources and spreads.

Dynamics of water vapour sorption on composite LiCl/(silica gel): An innovative configuration of the adsorbent bed

Adsorption Heat Conversion (AHC) is energy and environment saving alternative to conventional compression systems. The growth of specific power of AHC systems is a prerequisite for the wider spreading of AHC. To enhance the power, an innovative adsorbent bed configuration was suggested, namely, a compact adsorbent layer prepared by gluing to heat exchanger surface ready-made grains instead of uniform adsorbent coating. The main goal was a comparative study of water sorption dynamics on loose and glued grains of a composite LiCl/(silica gel) to evaluate the power enhancement. The grains were glued to aluminium foil with various binders, both organic and inorganic, the effect of the binder nature was studied. The effective heat transfer coefficient was evaluated under typical conditions of adsorption chilling cycle. The main findings are: (a) the effective heat transfer coefficient is increased up to 1.5 times when using inorganic binders; (b) organic binders do not affect heat transfer; (c) heat transfer intensification leads to accelerating initial stage of the ad/desorption; (d) at longer times, a desorption slowdown is observed due to hindered mass transfer or LiCl crystallisation. The factors controlling the adsorption kinetics are determined and recommendations are formulated for designing innovative bed configuration.

Experimental study of solution defrosting characteristics of air source heat pump

Air source heat pumps are widely used due to their high efficiency, energy-saving, and environmentally friendly nature. However, frost accumulation on heat exchanger surfaces restricts their application in regions with low temperatures and high humidity. Additionally, existing defrosting methods have certain limitations. To address these issues, this study presents a defrosting solution rooted in the chemical potential theory. The defrosting mechanism is first scrutinized through this theory. Subsequently, a system for air source heat pump solution defrosting is devised, along with a simulation test setup to explore defrosting characteristics utilizing sodium formate (HCOONa) solution. Outcomes reveal the efficacy of sodium formate in eliminating exchanger frost. Sustaining air circulation during defrosting could expedite frost layer removal and shorten the defrosting duration. Moreover, greater spray flow rate, temperature, and defrosting solution concentration correlate with reduced defrosting time. Notably, in this experiment, the defrosting amounts for the two experimental conditions were essentially the same at the time points of 90 s, 180 s, and 270 s at solution concentrations of 25% and 30%. This indicates that further increasing the solution concentration does not significantly accelerate the rate of frost melting.

Detection of frost growth and distribution on louver and offset strip fins of a microchannel heat exchanger using capacitance sensing approach

Frost growth and distribution on a microchannel heat exchanger (MCHX) are measured using a capacitance sensing approach. Capacitive sensors using two electrode layouts are developed to detect the real-time frost growth in the direction along and perpendicular to the airflow. Frosting and defrosting experiments are conducted under different air and coolant temperatures on both louver fins and rectangular offset strip (ROS) fins. The results demonstrate that the frost thickness is measured on the actual heat exchanger surface with an estimated uncertainty of ± 0.128 mm (95% confidence). A delay of frost growth on the trailing edge is captured by the sensor for both fin structures, and a larger frost thickness is obtained under lower air and coolant temperatures. The fin shape effect on the frost distribution profile is also characterized using the sensor. Compared to that on louver fin, the frost thickness at the leading edge of ROS fin is larger, as is the frost thickness difference between the leading and trailing edges. Due to the louver effect, the frost is more uniformly distributed over the fin. Based on the capacitance data at five locations, a frost distribution map is constructed and agrees well with frost imaging. This work demonstrates the development and application of a novel method for frost distribution detection for heat exchangers, providing a powerful tool for the study of the frosting process and defrost control.

Разработка общего критериального уравнения расчета теплоотдачи к углеводородным и неуглеводородным средам при их естественной конвекции в условиях локального осадкообразования

The paper presents a new general criterion equation for calculating heat transfer under conditions of natural convection of the hydrocarbon and non-hydrocarbon media with local deposit formation on the heat exchange surface. Information sources on the research topic were analyzed. Examples of deposits in certain power plants and jet engines are provided. Main factors influencing the carbon-containing deposits on the heated walls of the fuel and coolant supply channels are considered. The new equation for calculating heat transfer was obtained based on a new deposit formation similarity criterion created by the author of this article, which simultaneously takes into account both the thermal and electrochemical nature of this process. Values of the new electrochemical numbers used in the calculation and included in the deposit formation similarity criterion were found for the carbon-containing deposits in the kerosene media and for the salt deposits (NaCl salt). Results of the experimental studies were generalized in the TS-1 kerosene media, salt solution and air with local deposit formation on the heat exchange surface. Some generalization results are presented in the corresponding graphs.

Heat transfer analysis in a heat exchanger with two coaxial tubes for sustainable aseptic processing of foods

Thermal-based approaches in food sterilization techniques principally utilize the heating process until the desired sterilization level of the food can achieved. More specifically, aseptic processing allows for a more cost-effective and time-efficient method for preserving food without degrading the food quality. In this work, we propose a heat transfer analysis of a scraped surface heat exchanger with two coaxial tubes that provides a simplified prototype of a sustainable aseptic processing system. The heat exchanger is installed with scraped blades to allow a stirring mechanism that can only oscillate back and forth at certain angles called oscillating angles. In this simulation, water and solid food particles are put in the system. Moreover, simple regression models are used to find the relation between the mechanical properties of the stirring process with the heat transfer properties. Therefore, experimental data of the temperature and heat transfer coefficient during the heating are used for model fitting. The model fit shows that the angle, frequency, and velocity of the stirring affect the temperature trend and the heat flow rate inside the system. Establishing a comprehensive heat transfer modelling can help to improve the traditional sterilization approach to focus on optimizing energy efficiency and reducing environmental impacts.

Development of an integrated waste heat recovery system consisting of a thermoelectric generator and thermal energy storage for a propane fueled SI engine

In this paper, an integrated waste heat recovery system consisting of a thermoelectric generator and a latent heat thermal energy storage system was constructed to convert the wasted heat energy in a spark-ignition engine into useful energy. The TEG used in the integrated system has a three-layer structure formed of two cold-side heat exchangers placed on the upper and lower surfaces of the rectangular mid-exhaust heat exchanger. A total of 2x12 = 24 pieces of TEM were used between the lower and upper TEG_ec_hex and mid-TEG_ex_hex. For LHTES, a 3-layer heat exchanger consisting of a vertical copper tube placed in the middle of two cylindrical coils in a cylindrical tank with a diameter of 280 mm and a height of 350 mm was used. Three different configurations (Case_1, Case_2 and Case_3) were designed for the integrated system, in which exhaust gases, engine coolant, and HTF are used in different combinations. As a result, while the TEG_power is 69.395 W in Case_1, it decreases to 64.375 W in Case_2 and 62.417 W in Case_3. The total energy generated in the TEG at 7200 s is 500.27 kJ, 463.99 W, and 449.97 kJ for Case_1, Case_2, and Case_3, respectively. The stored energy in PCM_hex at 7200 s is 1372.91 kJ in Case_1, but it increases to 2255.16 kJ in Case_2 and to 2559.39 kJ in Case_3. While the fuel energy recovered by the integrated system is 1.03% in Case_1, it increases to 1.49% in Case_2 and 1.66% in Case_3.

A novel concept of enhanced direct-contact condensation of vapour- inert gas mixture in a spray ejector condenser

An analytical model of direct steam condensation (DCC) in the novel idea of spray ejector condenser (SEC) in the presence of inert gas has been developed. It is based on continuity, momentum and energy equations for the steam-carbon dioxide mixture and direct contact condensation mechanisms due to heat transfer and concentration. Crucial in the process of DCC is atomisation of the motive fluid in the ejector. The effect of atomised droplet size is exhibiting a significant amplification influence with increasing size of the droplet. Motive fluid is driving the secondary fluid-mixture of steam and inert gas in the Venturi nozzle and is cold enough to cause direct condensation. The intensity of heat transfer process from steam to water when the phases are in direct contact is much higher than the heat transfer intensity in surface heat exchangers. The analytical model pertains to a subcritical flow of a mixture of steam and gas in SEC. The model exhibits satisfactory agreement with experimental data. The model of DCC predicts higher values of temperature drop between inlet and outlet from the mixing section for the case of smaller steam-CO2 flow rates. Increasing the flow rate of steam mixture from 1.2 g/s to 3.6 g/s results in a reduction of steam mixture temperature from 25 °C to 14 °C respectively, at CO2 flow rate of 6.8 m3/h. Condensation without presence of CO2 in the same range of steam flow rate, i.e. from 1.2 g/s to 3.6 g/s results in reduction of steam mixture temperature from 56 °C to 25 °C respectively, confirming in such way the effect of CO2 presence on the efficiency of DCC. Such model allow for discussion of parameters affecting process of condensation in SEC and ability of application such condenser in power plants.

CFD MODELLING OF THERMAL AERODYNAMIC CHARACTERISTICS OF COMPOSITE FLAT-OVAL TUBE WITH INCOMPLETE CORRUGATED FIN

In the article thermo-aerodynamic characteristics of the original design of a heat exchange surface - a flat-oval tube with incomplete corrugated fins, which can be used for the modernization of heat exchange equipment, are studied. The purpose of the work is a preliminary assessment of the efficiency of flat-oval tubes with incomplete corrugated fins and obtaining an empirical dependence for calculation using CFD modeling. The object of the study is forced convection during transverse washing of a composite flat-oval tube with incomplete corrugated fins. The subject of the study is heat transfer and aerodynamics during transverse washing of a composite flat-oval tube with incomplete corrugated fins. In the work, the values of the dimensionless coefficient of heat transfer and pressure loss are obtained, as well as the calculated dependence for determining the coefficient of heat transfer and aerodynamic resistance of the mentioned types of tubes is presented. Data verification was carried out with a smooth tube and a tube with spiral fins. The scientific novelty of the work is the establishment of the fact that flat-oval tubes with incomplete corrugated fins have a higher heat transfer coefficient compared to smooth tubes (by 45 – 50 % in average) and spiral-ribbed tubes (by 25 – 30 % in average) with simultaneous increasing of aerodynamic resistance by 25 – 30 % at the same flow rate of the heat carrier. Practical application of the obtained results allows designers to increase the surface area of heat exchange, which makes these tubes prospective for using in power engineering and industry, in particular, for dry cooling towers of nuclear power plants and thermal power plants, for boiler heat exchangers, for gas turbine installations, etc.