Abstract: Metal foam heat exchangers have attracted a great deal of interest in numerous engineering fields due to their superior thermal capabilities. In the present study, the heat transfer characteristics of a double-pipe heat exchanger with metal foam insert are numerically investigated. The Forchheimer-extended Darcy equation and the local thermal non-equilibrium (LTNE) model are used to predict the fluid and energy transports, respectively. Thermal resistance of the interface solid wall is considered, while the porous-solid boundary follows the continuity principles. The commercial software FLUENT with specific user defined functions (UDFs) is adopted to implement the simulation. Configurations with uniform foam structure are firstly used to analyze the effects of flow arrangement, foam structural parameters (porosity and pore density) and thermal conductivity on the heat exchanger effectiveness and total pressure drop. Then, graded foam structure along the radius is proposed to further make use of the heat transfer potential of metal foam. The overall thermal performance with increasing and decreasing arrangements of porosity and pore density is assessed. The results indicate that the counter flow shows good performance, with 37.5% higher than the parallel flow in effectiveness. The effectiveness and total pressure drop present monotonic variation with the foam structural parameters for the uniform designs, while maximum performance factor occurs at 15 PPI. The effectiveness has a reduction after gradual increase to a peak 0.89 with the increasing of thermal conductivity of foam matrix. For the designs of graded foam structure, using lower porosity and small pore density at both side of the inner pipe wall shows better overall performance with the performance factors of 4.41 and 4.54.

Abstract: Since printed circuit heat exchangers (PCHE) are the largest modules of a supercritical carbon dioxide Brayton cycle, they can considerably affect the whole system’s performance and layout. Straight-channel and zigzag-channel printed circuit heat exchangers have frequently been analyzed in the standalone mode and repeatedly proposed for sCO2−BC. However, the impact of heat exchanger designs with straight and zigzag-channel configurations on the performance of the cycle and its components, i.e., the turbine and compressor, has not been studied. In this context, this study evaluates the effect of different heat exchanger designs with various values of effectiveness (ϵ), inlet Reynolds number (Re), and channel configuration (zigzag and straight channel) on the overall performance of the sCO2−BC and its components. For the design and analysis of PCHEs, an in-house PCHE design and analysis code (PCHE-DAC) was developed in the MATLAB environment. The sCO2−BC performance was evaluated utilizing an in-house cycle simulation and analysis code (CSAC) that employs the heat exchanger design code as a subroutine. The results suggest that pressure drop in PCHEs with straight-channel configuration is up to 3.0 times larger than in PCHEs with zigzag-channel configuration. It was found that a higher pressure drop in the PCHEs with straight channels can be attributed to substantially longer channel lengths required for these designs (up to 4.1 times than zigzag-channels) based on the poor heat transfer characteristics associated with these channel geometries. Thus, cycle layouts using PCHEs with a straight-channel configuration impart a much higher load (up to 1.13 times) on the recompression compressor, this in turn, results in a lower pressure ratio across the turbine. Therefore, the overall performance of the sCO2−BC using PCHEs with straight-channel configurations is found to be substantially inferior to that of layouts using PCHEs with zigzag-channel configurations. Finally, optimization results suggest that heat exchanger’s design with inlet Reynolds number and heat exchanger effectiveness ranging from 32 k to 42 k and 0.94>ϵ>0.87, respectively, are optimal for sCO2−BC and present a good bargain between cycle efficiency and its layout size.

Abstract: In micro heat exchangers, due to the presence of distributing and collecting manifolds as well as hundreds of parallel microchannels, a complete conjugate heat transfer analysis requires a large amount of computational power. Therefore in this study, a novel methodology is developed to model the microchannels as a porous medium where a compressible gas is used as a working fluid. With the help of such a reduced model, a detailed flow analysis through individual microchannels can be avoided by studying the device as a whole at a considerably less computational cost. A micro heat exchanger with 133 parallel microchannels (average hydraulic diameter of 200 μ m) in both cocurrent and counterflow configurations is investigated in the current study. Hot and cold streams are separated by a stainless-steel partition foil having a thickness of 100 μ m. Microchannels have a rectangular cross section of 200 μ m × 200 μ m with a wall thickness of 100 μ m in between. As a first step, a numerical study for conjugate heat transfer analysis of microchannels only, without distributing and collecting manifolds is performed. Mass flow inside hot and cold fluid domains is increased such that inlet Reynolds number for both domains remains within the laminar regime. Inertial and viscous coefficients extracted from this study are then utilized to model pressure and temperature trends within the porous medium model. To cater for the density dependence of inertial and viscous coefficients due to the compressible nature of gas flow in microchannels, a modified formulation of Darcy-Forschheimer law is adopted. A complete model of a double layer micro heat exchanger with collecting and distributing manifolds where microchannels are modeled as the porous medium is finally developed and used to estimate the overall heat exchanger effectiveness of the investigated micro heat exchanger. A comparison of computational results using proposed hybrid methodology with previously published experimental results of the same micro heat exchanger showed that adopted methodology can predict the heat exchanger effectiveness within the experimental uncertainty for both cocurrent and counterflow configurations.

Abstract: This paper introduces a kind of open cycle absorption heat wet flue gas heat recovery system, which use CaCl2 as the working medium. The system will use the wet heat recovery method and combined with an efficient heat pump system for flue gas as a heat source generator. Through direct contact with the solution in the absorber, the flue gas is going to carry out gas, liquid heat transfer between heat exchanger, realization of sensible heat and latent heat step by step. As the key part of the system, absorber is established by one-dimensional steady-state heat transfer and mass transfer model. This paper uses the finite difference method to model the discrete numerical methods, and analyzes the characteristics of heat and mass transfer in the absorber. We obtain the concentration curves of the three kinds of working medium's temperature and flow along the height direction. We also analyze the influence of CaCl2 solution parameters changes on the absorption process, parsing the reason of the temperature change by analyzing the three working medium's energy flow trend. We found that the temperature change of flue gas is non-monotonic, which decreases gradually in the range of absorption tower height 0–0.9 m, and then increases gradually. The reason for this change is that sensible heat exchange and latent heat exchange exist between flue gas and solution. Although such a change has an impact on the efficiency of the system, it prevents the "white smoke" from condensing in the air, which effectively protects the environment. Compared with conventional LiBr absorption heat pump, the system constructed in this paper has certain advantages in latent heat recovery, flue gas heat energy utilization, energy conservation and emission reduction and economy.

Abstract: Performance is analyzed for kinetic energy variation in high velocity stream heat exchangers that are subjected to external heat transfer. Analytical solutions are obtained using the method of inverse operators. They are verified against the reported expressions in the appropriate limits for constant kinetic energy system as well as for perfectly insulated conditions. Kinetic energy decay in hot stream enhances performance that exceeds the conventional heat exchanger effectiveness. For kinetic-to-thermal energy ratio and dimensionless characteristic length constant each equaling unity on the hot side, the terminal effectiveness under balanced operation is 136% for counter-flow arrangement and 82% for parallel-flow in the absence of external heat load. On the contrary, decay on the cold side lowers performance. For unbalanced flow, kinetic energy change in the higher heat capacity rate fluid has a lesser impact on the effectiveness. Thermal interaction with the ambient generally has a deleterious effect on the hot stream effectiveness of the cryogenic system. However, the performance improves under certain conditions such that the rise in fluid temperature caused by kinetic energy deterioration promotes heat loss to the surroundings.

Abstract: Nuclear power conversion in space has been approached by various means since the first space missions, with the advent of concepts such as thermoelectric, thermionic and thermodynamic conversion. Nowadays, thermal cycles are under greater focus for being capable of providing higher conversion efficiencies. In this context, one of the main concerns of engineers is the trade-off between power and mass. Therefore, this work aims the optimization of a recuperator used in a regenerative closed Brayton cycle applied for power conversion in the project of a small-scale nuclear reactor. The recuperator consists of a cross-flow, shell-and-tube heat exchanger with a matrix of tubes distributed in a staggered configuration. In this work, the number of tubes and the mass flow rate are varied. The number of tubes distributed axially is fixed as 4, whereas the quantity around the axis can be 5, 7, 9, 12 and 16 tubes. The working fluid considered in this study is a mixture of noble gases He-Xe with a molecular weight of 40 g/mol, whereas Inconel alloy 617 is applied as the recuperator material. The optimization procedure was based on the entropy generation minimization and the heat exchanger effectiveness, using the Computational Fluid Dynamics (CFD) technique to obtain the flow field. Optimum mass flow rates are obtained for all the geometries at the points of minimum entropy generation number, around which lie the ranges of tested mass flow rates. The ratio between the entropy generation number and effectiveness associated with the optimum mass flow rate is considered a performance evaluation criterion, and the dependence of this parameter with exchanger mass is assessed in order to select the most suitable geometry for the studied application. This analysis leads to the optimum design point at the geometry of 9 tubes around the recuperator axis, yielding a lost available work of 929.76 W for an ambient temperature of 298 K.

Abstract: Indirect two-phase cooling with refrigerants allows management of CPU/GPU heat loads well in excess of those that can be managed with water cooling. Two-phase mini channel boilers are at the heart of such systems. Similar to single-phase cold plates, the performance of mini-channel boilers depends on proper selection of design parameters such as fin pitch, thickness and total surface area that ultimately dictates the number of fins. But unlike single-phase cold plates, two-phase cold plates must also take into account ancillary issues such as boiling flow instabilities, vapor generation and potential loss of coolant that severely affect thermal performance. The traditional metric used in assessing the thermal performance of boilers is thermal resistance, which can be used for comparative studies but does not offer a true assessment of cooling limits. In this paper, we introduce a new definition of heat exchanger effectiveness for two-phase boilers that can be used to compare actual performance to maximum performance and by its definition can also establish cooling limits. We show that the thermal resistance is directly related to the effectiveness. By first maximizing effectiveness, the design thermal resistance can be achieved by the proper sizing of an optimized heat exchanger core. The methodology is first validated for single-phase liquid cold plates by comparing the thermal resistance obtained from the proposed approach to that obtained from the commonly used definition. The proposed approach is demonstrated on experimentally characterized boilers found in the literature. The experimentally determined effectiveness for these test boilers demonstrate s the utility o f the technique.

Abstract: In this research, the performance of counter flow double tube heat exchanger of 1m length, 19.0 mm outer diameter, and 9.5 mm inner diameter made from copper has been studied numerically using nano-water as a cold fluid. Al2O3 nanoparticles of 40 nm diameter with a volume concentration of 0.5% have been used with water as base fluid. The cold nano-water flows inside the inner tube at a volume flow rate of 3 Liter/min and 7 Liter/min, which enters the heat exchanger at 15 °C, whereas hot water flows in an annular space of the heat exchanger at a volume flow rate of 5 Liter/min. and enters the heat exchanger at a temperature of 50 C°. ANSYS FLUENT (2020 R1) was used to solve the governing differential equations and to estimate the heat exchanger effectiveness. The results obtained revealed an enhancement in the performance by using nanowater as a working fluid. The maximum heat exchanger effectiveness obtained when using nano water is 31%, for a volume flow rate of 7 Liter/min.

Abstract: This chapter is mainly focused on performance evaluation criteria for single-phase flow. The objectives and constraints of fixed geometry criteria, variable geometry criteria and fixed cross-sectional flow area criteria have been discussed here. The thermal resistance, St and f relations, heat exchanger effectiveness, effect of reduced exchanger flow rate and flow over finned tube bank topics have been presented.

Abstract: Heat exchangers are used in many applications ranging from power plants to air conditioning systems. Due to faster demand and growth of the thermal industry, the heat exchangers are under continuous development. By studying the effect of taper angle on the performance of cone shaped helical coil heat exchanger, the optimum angle for the highest rate of heat transfer can be found out. Due to this, the best taper angle can be implemented in cone shaped helical heat exchanger which will provide a high rate of heat transfer and compact size of the heat exchanger. In this work, the heat exchanger with a helical coil, a coil with 80° inclination, a coil with 70° inclination and coil with 60° inclinations are used for the study of heat exchanging process. In this study, the amount of heat transfer between cold water and hot water is studied with the change in the taper angle of the conical coil. It is observed that the angle of the coil has a positive influence on the amount of heat transfer in the heat exchanger. The overall heat transfer coefficient of the heat exchanger is higher for conical heat exchangers and it increases with the decrease in the inclination angle and Heat exchanger effectiveness of the heat exchanger increases with the conical heat exchanger.

Abstract: Earth Air Heat Exchanger (EAHE) system is a Passive Cooling technique which uses earth’s potential to maintain a constant temperature throughout the year at certain depths as a source or sink to cool air during summer and vice versa in winter for space air-conditioning application. EAHE systems are best suited in hot and dry climatic conditions because EAHE systems in humid areas face challenges of condensation of water and contamination by microorganisms. Commercial Heating, Ventilation, and Air Conditioning (HVAC) systems are now established everywhere because there are standard methods for their calculations and manpower for installation. EAHE systems, being in their early stages lack such standards, installations guidelines and hence have less commercialization. One such effort has been made in this paper to design an EAHE system for a classroom in a hot and dry climate of Nagpur City, India. Heat exchanger calculations were made based on the NTU method, air flow rates were referred from ASHRAE ventilation standard and a piping layout is created so as to use the designed system for commercial purposes.

Abstract: The application of energy recovery system has been proven as one of the key solutions to produce energy savings and to provide fresh outdoor air in building ventilation. The performance of the system can be evaluated in terms of efficiency and recovered energy (heat and mass transfer) through its heat exchanger. The efficiency can be determined using ASHRAE standard, effectiveness–NTU method and global efficiency. Meanwhile, recovered energy is calculated based on the heat and mass transfer rates of the system. This chapter provides a background of the performance evaluation of energy recovery system from existing established data and previous works in the literature.

Abstract: To achieve an improvement of heat exchanger effectiveness (heat transfer rate) without any increase of a fan motor output. The gas heated by the heating units or the gas cooled down by the cooling units is sent to the blowing nozzles 2 by a fan. Then, the blowing nozzles 2 blow the gas sent by the fan through their outlets. Each of the outlets has a non-circular planar shape with a projection portion thereof being projected inwardly. This allows a shape of the gas in cross section perpendicular to a direction where the gas is blown through the outlet of each of the blowing nozzles 2 to be changed by the projection portion with time (switching phenomenon) . Such a stitching phenomenon enables to be increased the heat exchanger effectiveness (heat transfer rate) on the printed board even if any output of the fan motor for rotating the fan does not increase.

Abstract: Data centres are complex energy demanding environments. The number of data centres and thereby their energy consumption around the world is growing at a rapid rate. Cooling the servers in the form of air conditioning forms a major part of the total energy consumption in data centres and thus there is an urgent need to develop alternative energy efficient cooling technologies. Liquid cooling systems are one such solution which are in their early developmental stage. In this article, the use of Computational Fluid Dynamics (CFD) to further improve the design of liquid-cooled systems is discussed by predicting temperature distribution and heat exchanger performance. A typical 40 kW rack cabinet with rear door fans and an intermediate air–liquid heat exchanger is used in the CFD simulations. Steady state Reynolds-Averaged Navier–Stokes modelling approach with the RNG K-epsilon turbulence model and the Radiator boundary conditions were used in the simulations. Results predict that heat exchanger effectiveness and uniform airflow across the cabinet are key factors to achieve efficient cooling and to avoid hot spots. The fundamental advantages and limitations of CFD modelling in liquid-cooled data centre racks were also discussed. In additional, emerging technologies for data centre cooling have also been discussed.

Abstract: Heat exchangers are widely used for heat recovery purposes in many industrial applications such as gasification systems. In a biomass gasification system situated at Melani village in Eastern Cape of South Africa, a significant quantity of heat energy is lost during syngas cooling. Thus, a heat exchanger was constructed and installed in the gasification system for the purpose of heat recovery. Therefore, the aim of this study is to evaluate the performance of the heat exchanger under variable operating conditions for counterflow and parallel flow configurations. The experimental investigation was carried out on a double pipe heat exchanger as the downdraft gasifier system operated on a wood consumption rate of 180 kg/h. The heat exchanger was installed at the exit point of the syngas in the gasifier, and water served as the cooling fluid. Inlet and outlet temperatures of the hot syngas and cooling water (fluids) were measured using thermocouples at variable flow rates. Experimental data were processed using energy equations to determine vital performance parameters (overall heat transfer coefficient, effectiveness, and log mean temperature difference). The findings showed that optimum heat exchanger effectiveness of 0.55 was determined at a mass flow rate of 0.07 kg/s. In addition, counterflow configuration was found to be approximately 14% more effective than the parallel flow configuration. This is attributed to the relative direction of the fluids in the configurations of both flows. The study recommends that double pipe heat exchanger is suitable for recovering heat from the gasification system.

Abstract: The heat transfer characteristic of heat exchangers is one of the key factors affecting the optimal dispatch of combined heat and power systems. In order to optimize the dispatch of the combined heat and power systems, a three-stage heat transfer model of the extraction steam is established based on the heat transfer principle to describe the heat transfer process of the heat exchanger, and the iteration method is used to calculate the extraction steam under a certain heat load. And then, a joint dispatch model of cogeneration power, thermal power and wind power is proposed with consideration of the heat transfer characteristics of the heat exchangers. Finally, with the minimum coal consumption as the optimization goal, an analysis is made of the influence of the heat exchanger’s heat transfer characteristics on the steam extraction heat transfer process and the combined heat and power system dispatch results. The case study shows that increasing the heat transfer area of the heat exchanger and reducing the water temperature at the outlet of the heat exchanger can improve the wind power consumption capacity to different degrees, and reduce the operating cost. The research results of this paper can provide heat transfer parameters for optimizing the dispatch scheme of the combined heat and power systems.