Heat Exchangers as Equipment and Integrated Items in Waste and Biomass Processing

Heat recovery systems play an important role in waste to energy and biomass processing. An efficient approach that follows a recommended hierarchy in design, process as a whole (e.g., incineration) → subsystem of the process (e.g., heat recovery system) → equipment (e.g., air pre-heater), is shown. Important factors have to be taken into consideration in processes for incineration (combustion of biomass), especially available energy, specific features of hot process fluid (flue gas), type of waste/biomass, fouling, and environmental impact. A combination of intuitive design, know-how, and a sophisticated approach based on up-to-date computational tools is shown. Some novel types of heat exchangers (e.g., air preheaters for high- and low-temperature applications, heat recovery steam generators and/or heaters, and those for specific applications) that can be substituted for conventional ones are presented. An improved or even optimum design of heat exchangers requires computational support in the following areas: a simulation based on energy and mass balance, the thermal and hydraulic calculation of heat exchangers, a CFD (computational fluid dynamics) approach, optimization, and heat integration. Some examples are presented. An approach that ranges from an idea to an industrial application is demonstrated on the novel design of integrated compact equipment (combustion chamber installed inside heat exchanger) for the thermal treatment of waste gases, including heat recovery. This approach involves simulation for obtaining basic process parameters, thermal and hydraulic calculations, design of experimental facility, the manufacture of the equipment and building of this facility, operation and functionality testing, data acquisition for validating and improving the CFD model, and the utilization of feedback from industrial applications.     

Computational Fluid Dynamics in Research and Design of Heat Exchangers

This paper gives a brief summary of computational methods in heat transfer equipment, such as CFD (computational fluid dynamics), methods for thermal problems (computational heat transfer), and turbulence modeling for single-phase applications in the design, research and development of heat exchangers. Details of the finite volume method and their extension to arbitrary geometries are presented. An overall presentation of turbulence modeling is provided. Some of the most common commercially available computer codes are listed. Ways to apply CFD to heat exchangers are discussed. The paper presents results from some examples of application of CFD methods for real heat exchangers in order to demonstrate how such methods can be used in the research, development, and design of heat transfer equipment. Limitations and shortcomings as well as needs for further research are also highlighted.     

Robust methodology for determination of heat transfer coefficient distribution in convection

To predict the microstructures, residual stresses and distortions in the heat treated metal components, it is important to accurately know the heat transfer coefficients (HTCs) between the hot work piece and cooling media. In this paper, a new method is presented to accurately determine the node-based HTC distribution by coupling computational fluid dynamics (CFD) with optimal weight functions and scale factors. With this new method, the predicted temperature profile of the work piece during quenching (rapid cooling) is in excellent agreement with experimental measurements. This new method can be also applied to accurately predict convection heat transfer in thermal equipment such as heat exchangers and refrigerators, building thermal design and other heat transfer related situations.     

Off-highway heavy-duty truck under-hood thermal analysis

A full-size of machine-level, under-hood thermal analysis has been conducted using CFD. In particular, an iterative approach has been developed and was used to speed up the convergence of the numerical solution of the energy equation, where the large heat source term poses a computational challenge. The Reynolds Averaged Navier–Stokes (RANS) method was applied to simulate under-hood turbulent flow, while a net-radiation enclosure model was included in the energy equation. The numerical results are used to verify and validate the concept design and prototyping of off-highway heavy-duty trucks, including parameters and components such as: the speed of cooling fans; core size of the heat exchangers; temperature; pressure drop; thickness of insulation and design options of the heat shield. Another objective is to benchmark a one-dimensional lumped in-house tool for industrial cooling system concept design and analysis. The CFD method is a powerful tool that can identify possible design drawbacks, evaluate design options, improve the heat transfer efficiency and provide optimal engineering solutions and strategies for complicated heavy-duty thermal management.     

Computational fluid dynamic simulations in thermal waste treatment technology—Design, optimisation and troubleshooting

The present work provides three case studies featuring applications of computational fluid dynamics (CFD) in the area of thermal waste treatment (TWT). The purpose of the paper is to demonstrate the benefits offered by simulations in TWT technology in the design, optimisation and troubleshooting of those systems. The case studies deal with practical problems, namely a performance evaluation of a fabric filter bag with an add-on Venturi nozzle, the design optimisation of flow homogenising vanes in a heat exchanger, and finally troubleshooting in a novel volatile organic compound (VOC) treatment unit.

Each case study includes an outline of the modelling approach and summary of the most important results. The flow analyses are performed using standard methods implemented in the commercial software code FLUENT. The case study on design optimisation combines decision support in the selection of the best conceptual solution and an automatic shape optimisation using the commercial code SCULPTOR. The results clearly show the usefulness and applicability of the CFD computations in the area of thermal waste treatment.     

Computational fluid dynamic investigation of liquid rack cooling in data centres

Relying on thermal air management in a data centre is becoming less effective as heat densities from the Information Technology (IT) equipment continue to rise. Direct liquid cooling is more efficient at transferring the waste heat, but requires liquid loops passing as close as possible to the heat source. A new Computational Fluid Dynamics (CFD) strategy is developed for data centre scenarios where a liquid loop heat exchanger is attached at the rear of server racks (back doors), which can avoid the need to separate the cold and hot air streams in traditional hot/cold aisle arrangements. The effectiveness of additional fans in the back door heat exchangers is investigated using the three-dimensional CFD model of a simplified three-aisle, six-rack data centre configuration.     

Numerical study of heat pipe application in heat recovery systems

Heat pipes are two-phase heat transfer devices with extremely high effective thermal conductivity. They can be cylindrical or planar in structure. Heat pipes can be embedded in a metal cooling plate, which is attached to the heat source, and can also be assembled with a fin stack for fluid heat transfer. Due to the high heat transport capacity, heat exchangers with heat pipes have become much smaller than traditional heat exchangers in handling high heat fluxes. With the working fluid in a heat pipe, heat can be absorbed on the evaporator region and transported to the condenser region where the vapour condenses releasing the heat to the cooling media. Heat pipe technology has found increasing applications in enhancing the thermal performance of heat exchangers in microelectronics, energy and other industrial sectors.Utilisation of a heat pipe fin stack in the drying cycle of domestic appliances for heat recovery may lead to a significant energy saving in the domestic sector. However, the design of the heat pipe heat exchanger will meet a number of challenges. This paper presents a design method by using CFD simulation of the dehumidification process with heat pipe heat exchangers. The strategies of simulating the process with heat pipes are presented. The calculated results show that the method can be further used to optimise the design of the heat pipe fin stack. The study suggests that CFD modelling is able to predict thermal performance of the dehumidification solution with heat pipe heat exchangers.     

Numerical Investigation on Thermal Performance Design of Cryogenic Compact Heat Exchangers with Serrated-Fin Channels

Abstract

Traditional empirical formulas of Colburn heat transfer factors will lead to a design deviation for cryogenic heat exchangers. This paper employs the computational fluid dynamics (CFD) technique to numerically study the thermal performance of cryogenic compact heat exchangers (CCHEs). To obtain more precise convective heat transfer coefficients, the heat transfer performance of CCHE with serrated fin channels is analyzed considering various cryogenic fluid properties, fin materials and the axial heat conduction (AHC), and a heat transfer deterioration rate is proposed to investigate the effect of AHC on the heat transfer performance of CCHEs. For the simulation design, a quasi-one-dimensional calculation model is developed to obtain the temperature and pressure fields of the whole heat exchanger using the previous CFD results of the finned channels to avoid the deviation caused by traditional empirical formulas. Finally, a case study for a CCHE in a practical system is designed and analyzed by the proposed approach. The results suggest that cryogenic conditions have a significant effect on the design performance of heat exchangers, especially when considering the influences of fluid properties, materials, and AHC. For different cryogenic fluids, accurate heat transfer factors should be selected for the design calculations, and materials with high thermal conductivity will increase the effect of AHC and deteriorate the performance of the CCHE.     

Thermal design of multi-stream heat exchangers

The thermal design of multi-stream heat exchangers of the plate and fin type is presented. Although originally used in low temperature processes, their application is extrapolated to above temperature processes and it is shown that, conceptually, multi-stream exchangers could replace whole heat recovery networks. The approach is based on the use of temperature vs. enthalpy diagrams or composite curves, which show how a multi-stream exchanger can be subdivided into block sections that correspond to enthalpy intervals and indicate the entry and exit points of the streams. A design methodology for plate and fin exchangers in countercurrent arrangement, characterized by the maximization of allowable pressure as a design objective is extended to the design of multi-fluid exchangers. The methodology uses a thermo-hydraulic model which relates pressure drop, heat transfer coefficient and exchanger volume. The potential applicability of the methodology is demonstrated on a case study.     

Alternative Approach in Optimization of Plate Type Heat Exchangers

Plate type heat exchangers are widely used in process industries for gas/gas applications. Typically, these exchangers prove to be very efficient, especially as air preheaters in process furnaces or in equipment used in environmental protection (e.g., in units for thermal disposal of wastes).

For economic reasons, there is a need for a new optimization approach for plate type heat exchanger design and operation. The objective function is to achieve a minimal total annual cost of heat exchangers. Pressure drop and heat transfer are interdependent, and both of them strongly influence capital and operating costs of any heat transfer system. In designing a heat exchanger, it is necessary to determine the optimal dimensions of the apparatus with the given conditions of the equipment operation.

The goal is to obtain the most economically optimal design. An economic assessment allows a comparable estimation of various alternatives. The total annual cost consisting of fixed and variable costs of the heat exchanger was selected as a criterion that summarizes different factors of influence into one objective function. Major cost components of a heat exchange system are as follows: capital, operating and maintenance costs of air and flue gas fans, and capital and maintenance costs of the plate type heat exchanger.

The application of the developed optimization approach is demonstrated through practical industrial examples.     

A review of the applications of heat pipe heat exchangers for heat recovery

The waste heat recovery by heat pipes is accepted as an excellent way of saving energy and preventing global warming. This paper is a literature review of the application of heat pipes heat exchangers for the heat recovery that is focused on the energy saving and the enhanced effectiveness of the conventional heat pipe (CHP), two-phase closed thermosyphon (TPCT) and oscillating heat pipe (OHP) heat exchangers. The relevant papers were allocated into three main categories, and the experimental studies were summarized. These research papers were analyzed to support future works. Finally, the parameters of effectiveness of the CHP, TPCT and OHP heat exchangers were described. This review article provides additional information for the design of heat pipe heat exchangers with optimum conditions in the heat recovery system.     

Velocity distribution and vibration excitation in tube bundle heat exchangers

Design criteria for tube bundle heat exchangers, to avoid fluidelastic instability, are based on stability criteria for ideal bundles and uniform flow conditions along the tube length. In real heat exchangers, a non-uniform flow distribution is caused by inlet nozzles, impingement plates, baffles and bypass gaps. The calculation of the equivalent velocities, according to the extended stability equation of Connors, requires the knowledge of the mode shape and the assumption of a realistic velocity distribution in each flow section of the heat exchanger. It is the object of this investigation to derive simple correlations and recommendations, (1) for equivalent velocity distributions, based on partial constant velocities, and (2) for the calculation of the critical volume flow in practical design applications. With computational fluid dynamic (CFD) programs it is possible to calculate the velocity distribution in real tube bundles, and to determine the most endangered tube and thereby the critical volume flow. The paper moreover presents results and design equations for the inlet section of heat exchangers with variations of a broad range of geometrical parameters, e.g., tube pitch, shell diameter, nozzle diameter, span width, distance between nozzle exit and tube bundle.     

CFD analysis of two types of welded plate heat exchangers

A numerical and experimental study has been carried out on the heat transfer and fluid flow in two types of welded plate heat exchangers (PHEs). A new approach providing a proper clearance between two adjacent corrugated plates is proposed to improve the mesh quality around the contact points. The clearance value and the influence on the mesh quality and computational results are carefully studied. The results show that a relative clearance of 0.02 is proper. The computational fluid dynamics (CFD) results agree with the experimental results with a deviation of 15%. The proposed approach is proved effective and practical because it can increase the grid quality without losing the accuracy of the results. This paper shows that CFD is a reliable tool for studying the effects of various geometrical configurations on the optimum design of a PHE.     

Optimization of micro heat exchanger: CFD,analytical approach and multi-objective evolutionary algorithms

Advances in miniaturization have led to the use of microchannels as heat sinks in industry. Studies have established that the thermal performance of a microchannel depends on its geometric parameters and flow conditions. This paper describes two approaches for determining the optimal geometric parameters of the microchannels in micro heat exchangers. One approach combines CFD analysis with an analytical method of calculating the optimal geometric parameters of micro heat exchangers. The second approach involves the usage of multi-objective genetic algorithms in combination with CFD.     

Comparative assessment of alternative cycles for waste heat recovery and upgrade

Thermally activated systems based on sorption cycles, as well as mechanical systems based on vapor compression/expansion are assessed in this study for waste heat recovery applications. In particular, ammonia-water sorption cycles for cooling and mechanical work recovery, a heat transformer using lithium bromide-water as the working fluid pair to yield high temperature heat, and organic Rankine cycles using refrigerant R245fa for work recovery as well as versions directly coupled to a vapor compression cycle to yield cooling are analyzed with overall heat transfer conductances for heat exchangers that use similar approach temperature differences for each cycle. Two representative cases are considered, one for smaller-scale and lower temperature applications using waste heat at 60 °C, and the other for larger-scale and higher temperature waste heat at 120 °C. Comparative assessments of these cycles on the basis of efficiencies and system footprints guide the selection of waste heat recovery and upgrade systems for different applications and waste heat availabilities. Furthermore, these considerations are used to investigate four case studies for waste heat recovery for data centers, vehicles, and process plants, illustrating the utility and limitations of such solutions. The increased implementation of such waste heat recovery systems in a variety of applications will lead to decreased primary source inputs and sustainable energy utilization.     

Characteristics of medium deep borehole thermal energy storage

Seasonal energy storage is an important component to cope with the challenges resulting from fluctuating renewable energy sources and the corresponding mismatch of energy demand and supply. The storage of heat via medium deep borehole heat exchangers is a new approach in the field of Borehole Thermal Energy Storage. In contrast to conventional borehole storages, fewer, but deeper borehole heat exchangers tap into the subsurface, which serves as the storage medium. As a result, the thermal impact on shallow aquifers is strongly reduced mitigating negative effects on the drinking water quality. Furthermore, less surface area is required. However, there are no operational experiences, as the concept has not been put into practice so far. In this study, more than 250 different numerical storage models are compared. The influence of the characteristic design parameters on the storage system's behaviour and performance is analysed by variation of parameters like borefield layout, fluid inlet temperatures and properties of the reservoir rocks. The results indicate that especially larger systems have a high potential for efficient seasonal heat storage. Several GWh of thermal energy can be stored during summertime and extracted during the heating period with a high recovery rate of up to 83%. Medium deep borehole heat exchanger arrays are suitable thermal storages for fluctuating renewable energy sources and waste heat from industrial processes. Copyright © 2016 John Wiley & Sons, Ltd.     

A CFD Study on the Effect of Shrinking Box Size on Cooling Airflows in Compact Electronic Equipment—The Case of Portable Projection Display Equipment

This work is concerned with the thermal design of portable projection display equipment (i.e., a portable projector). The portable projector is being made smaller and lighter to better serve the carrier, while its luminance is enhanced to benefit the viewer. The confluence of these demands increases the thermal loading on the projector. The design of cooling airflow path is becoming evermore important; however, the complex internal organization of the projector hampers an attempt to conduct flow and heat transfer analysis. A particularly important issue for the projector designer is to predict the effect of shrinking box size of the next generation product on cooling airflow. This paper describes a way to foresee the effects of reducing the key geometric dimensions of the projector on the distribution of airflow. The analysis is conducted using a computational fluid dynamics (CFD) code. CFD results are obtained on sampled points in the parameter domain and used to find the effects of various geometric parameters on the airflow rates. The sampling and screening of CFD results follows the Taguchi method (or the method of experimental design). The most influential parameters for the total airflow rate and the airflow rate over the electronics board are identified. This conclusion, however, is only an illustration of how the CFD analysis can assist the equipment designer in planning the next generation product. The methodology reported here can be applied in the plan and design of other electronic equipment that have similarly complex internal organizations in shrinking system boxes.     

CFD simulation of a Gifford–McMahon type pulse tube refrigerator

Pulse tube refrigerator has the advantages of long life and low vibration over the conventional cryocoolers, such as Gifford–McMahon (GM) and Stirling coolers because of the absence of moving parts in low temperature. This paper performs a two-dimensional computational fluid dynamic (CFD) simulation of a Gifford–McMahon type double inlet pulse tube refrigerator (DIPTR), operating under a variety of thermal boundary conditions. A commercial Computational Fluid Dynamics (CFD) software package Fluent 6.1 is used to model the oscillating flow inside a pulse tube refrigerator. Helium is used as working fluid for the entire simulation. The simulated DIPTR consists of a transfer line, an after cooler, a regenerator, a pulse tube, a pair of heat exchangers for cold and hot end, an orifice valve with connecting pipe, a double inlet valve with connecting pipe and a reservoir. The simulation represents fully coupled systems operating in steady-periodic mode. The externally imposed boundary condition is sinusoidal pressure inlet by user defined function at one end of the tube and constant temperature or heat flux boundaries at the external walls of the hot end and cold-end heat exchangers. The general results, such as the cool down behaviors of the system, phase relation between mass flow rate and pressure at pulse tube section and the temperature profile along the wall of the cooler are presented.The simulation shows the minimum decrease in temperature at cold-end heat exchanger for a particular combination of cryocooler assembly. The CFD simulation results are compared with available experimental data. Comparisons show that there is a reasonable agreement between CFD simulation and experimental results.     

A computational fluid dynamics and finite element analysis design of a microtubular solid oxide fuel cell stack for fixed wing mini unmanned aerial vehicles

Computational fluid dynamics (CFD) and finite element analysis (FEA) are important modelling and simulation techniques to design and develop fuel cell stacks and their balance of plant (BoP) systems.The aim of this work is to design a microtubular solid oxide fuel cell (SOFC) stack by coupling CFD and FEA models to capture the multiphysics nature of the system. The focus is to study the distribution of fluids inside the fuel cell stack, the dissipation of heat from the fuel cell bundle, and any deformation of the fuel cells and the stack canister due to thermal stresses, which is important to address during the design process. The stack is part of an innovative all-in-one SOFC generator with an integrated BoP system to power a fixed wing mini unmanned aerial vehicle. Including the computational optimisation at an early stage of the development process is hence a prerequisite in developing a reliable and robust all-in-one SOFC generator system. The presented computational model considers the bundle of fuel cells as the heat source. This could be improved in the future by replacing the heat source with electrochemical reactions to accurately predict the influence of heat on the stack design.     

Heat transfer design in adsorption refrigeration systems for efficient use of low-grade thermal energy

Adsorption refrigeration and heat pump systems have been considered as important means for the efficient use of low-grade thermal energy of 60–150 °C. Sorption systems are merely thermodynamic systems based on heat exchangers, and therefore a good design to optimize heat and mass transfer with reaction or sorption processes is very important, for which the notable technique is the use of expanded graphite to improve both heat and mass transfer in the chemisorption beds. Studies have also shown the need to enhance the heat transfer in adsorption bed by matching with the efficient heat transfer of thermal fluids. Heat pipes and good thermal loop design coupled with adsorption beds could yield higher thermal performance of a sorption system. A novel design with passive evaporation, known as rising film evaporation coupled with a gravity heat pipe was introduced for high cooling output. It has also been shown that the performance of traditional heat and mass recovery in the sorption systems is limited, and novel arrangement of thermal fluid and refrigerant may improve the performance of sorption systems. Based upon the above researches, various sorption systems have been developed, and high performances have been reached.