Heat Exchanger Core Research Articles

Homogenized Elastic Stiffness of Fin Layer in Tube-Fin Structures (Evaluation and Verification)

This paper describes the homogenization of fin layers for structural analysis of heat exchanger cores composed of flat tubes and wavy fins. It is assumed that uniform deformation prevails at the bonded interfaces between fins and tubes, and also that the fins have the Y-periodicity in the fin layers. The homogenization method for 3D periodic solids is then shown to be applicable to the homogenization of fin layers, provided that the uniform deformation condition is imposed at the bonded interfaces between fins and tubes. The resulting homogenization method is applied to designed and real shapes of outer and inner fins in an intercooler. The evaluated, homogenized elastic stiffnesses of fin layers are verified by performing finite element analyses using full-scale and fin-homogenized models of tube-fin connected pieces subjected to uniaxial compression. An experiment of a tube-fin laminated block under uniaxial compression is further performed to demonstrate that the real shapes of outer and inner fins are definitely important for the homogenization of fin layers.

Fin Effects in Flow Channels of Plate-Fin Compact Heat Exchanger Cores

The fin effects on laminar forced convection of air in the interfin passages of plate-fin heat exchangers are investigated. Steady state fully developed flows in rectangular, trapezoidal, and triangular plate-fin channels are considered. With H1 and T conditions at the partition plates, the conjugate conduction–convection fin problem is solved computationally. The fin effects on the convective Nusselt number are shown to scale by a new dimensionless parameter Ω, which accounts for the attendant fin material and size; its limits describe perfectly conducting and nonconducting fins. Ineffective fins and the consequent reduction in the convective heat transfer coefficient are most pronounced in low fin density cores with longer fins in low-conductivity metal (stainless steel). However, with increasing fin density and shorter fins, the convection performance is virtually the same as that with 100% fin efficiency; the same is the case when fins are made of very high conductivity metal (copper). These results provide design insights for optimizing the conjugate fin-conduction and fluid-flow convection performance in plate-fin compact heat exchangers.

Safety analysis of the reference research reactor MTR during reactivity insertion accident using the code MERSAT

Abstract The IAEA’s reference research reactor MTR-10 MW has been modeled using the code MERSAT. The developed MERSAT model consists of detailed representation of primary and secondary loops including reactor pool, bypass, main pump, heat exchanger and reactor core with the corresponding neutronics and thermalhydraulic characteristics. Following the successful accomplishment of the steady state operation at nominal power of 10 MW, reactivity insertion accident (RIA) for three different initial reactivity values of $1.5/0.5 s, $1.35/0.5 s and $0.1/1.0 s have been simulated. The predicted peaks of reactor power, hot channel fuel, clad and coolant temperatures demonstrate inherent safety features of the reference MTR reactor. Only in case of the fast RIA of $1.5/0.5 s, where the peak power of 133.66 MW arrived 0.625 s after the start of the transient, the maximum hot channel clad temperature arrives at the condition of subcooled boiling with the subsequent void formation. However, due to the strong negative reactivity feedback effects of coolant and fuel temperatures the void formation persists for a very short time so that thermalhydraulic conditions remained far from exceeding the safety design limits of thermalhydraulic instability and DNB. Finally, the simulation results show good agreement with previous international benchmark analyses accomplished with other qualified channel and thermalhydraulic system codes.

A Calculation Method for the Optimized Thermal-Fluid Dynamic Sizing of Heat Exchangers

A thermal-fluid dynamic calculation model has been developed for sizing some compact heat exchangers. In particular, the exhaust heat recovery systems are considered, as the internal regeneration considerably improves the turbogas power cycles efficiency. The calculation model is designed for selecting the best heat transfer surfaces and optimizing some objective-functions. For almost 60 plate-fin heat transfer surfaces, analytical correlations are derived in a database-like form, to calculate the Colburn j factor and Fanning f factor as functions of Reynolds number. A model is developed also for innovative PCHEs, using a simplified approach. The sizing procedure, based on the core mass velocity equation, is implemented on an electronic worksheet. If the minimum heat exchanger effectiveness and the maximum pressure drops are input, the model gives the heat exchanger core sizing for the possible combinations of the heat transfer surfaces. For each combination, the calculation method minimizes (or maximizes) the selected objective-function by means of an optimization procedure, performed by a solver through the Newton or the conjugate gradients algorithm. The thermal-fluid dynamic calculation method is applied to a high-temperature recuperator with a high-streams pressure ratio. The results show the difficulty of arranging streams with highly different volumetric flows (i.e., same massive flow rates but very different pressures) on the two sides of the heat exchanger, so that very unbalanced aspect ratios arise for the cores, due to the limits imposed to the maximum allowable pressure drops.

Simplified modelling of the behaviour of 3D-periodic structures such as aircraft heat exchangers

In this paper, experimental, analytical and numerical analysis are used to study and model the mechanical behaviour of a heat exchanger core consisting of a 3D-periodic structure. The purpose of the present investigation is not only to acquire knowledge on the mechanical behaviour of a given heat exchanger core but also to propose a simplified approach to model this behaviour. An experimental study is carried out in order to get an insight on the mechanical behaviour of this structure. Global static characteristics are obtained via analytical and finite element analysis of a unit cell of the core. Dynamic behaviour is studied by means of finite element calculations based on the results of the static modelling. The proposed approach is validated by comparison with experimental tests results.

CFD Simulation of Flow Distribution in the Header of Plate‐Fin Heat Exchangers

AbstractThe flow distribution through a plate‐fin heat exchanger is studied by using a computational fluid dynamics (CFD) code, FLUENT. The flow distribution through any heat exchanger affects its performance. In designing a heat exchanger, it is assumed that the fluid is uniformly distributed through the heat exchanger core. In practice, however, it is impossible to distribute fluid uniformly, because of an improper inlet configuration, imperfect design, and a complex heat transfer process. The CFD simulation of the flow distribution in the header of a conventional plate‐fin heat exchanger is presented. It is found that the flow maldistribution is very serious in the y‐direction of the header. A modified header is proposed and simulated using CFD. The modified header configuration has a more uniform flow distribution than the conventional header configuration. Hence, the efficiency of the modified heat exchanger is seen to be higher than that of the conventional heat exchanger.

Experimental investigation of header configuration improvement in plate–fin heat exchanger

Flow characteristics in the header of a plate–fin heat exchanger have been investigated by means of Particle Image Velocimetry (PIV). A series of velocity vector and streamline graphs of different cross-sections are obtained in the experiment. The experimental results indicate that performance of fluid maldistribution in a conventional header is very serious, while the improved header configuration with a punched baffle can effectively enhance the uniformity of flow distribution. The flow maldistribution parameter and the ratio of the maximum velocity to the minimum in a plate–fin heat exchanger decreases by installing the punched baffle. Further heat exchange experiments indicate that the temperature is distributed more uniformly in the improved heat exchanger core and the heat exchanger effectiveness can be effectively enhanced. The conclusion of this paper is of great significance in the optimum design of plate–fin heat exchanger.

Effect of the plate thermal resistance on the heat transfer performance of a corrugated thin plate heat exchanger

AbstractTwo‐dimensional conjugate conduction/convection numerical simulations were carried out for flow and thermal fields in a unit model of a counter‐flow‐type corrugated thin plate heat exchanger core. The effects of the thermal resistance of the solid plate, namely the variation of the plate thickness and the difference of the plate material, on the heat exchanger performance were examined in the Reynolds number range of 100<Re<400. Higher temperature effectiveness was obtained for a thicker plate at any Reynolds number, which was a unique feature of corrugated thin plate geometry. Detailed discussions on the thermal fields revealed that restricting the heat conduction along the plate by making the plate thinner or choosing a low thermal conductivity material causes a larger plate temperature variation along the plate, and, consequently, a smaller amount of thermal energy exchanged between two fluids. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(3): 209–223, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20110

Printed circuit heat exchanger thermal–hydraulic performance in supercritical CO 2 experimental loop

Heat transfer and pressure drop characteristics of the Printed Circuit Heat Exchanger (PCHE) were investigated in an experimental supercritical CO 2 loop. The inlet temperature and pressure were varied from 280 to 300 °C/2.2 to 3.2 MPa in the hot side and from 90 to 108 °C/6.5 to 10.5 MPa in the cold side while the mass flow rate was varied from 40 to 80 kg h −1. The overall heat transfer coefficient range is 300–650 W m −2 K −1 while the compactness with respect to the heat exchanger core is approximately 1050 m 2 m −3. The empirical correlations to predict the local heat transfer coefficient and pressure drop factor as a function of the Reynolds number have been proposed for the tested PCHE.

Dendritic counterflow heat exchanger experiments

In this paper we report experimentally the hydraulic and thermal behavior of a balanced counterflow heat exchanger in which each stream flows through a tree-shaped structure covering a circular area. The tree structure is the same on both sides of the heat exchanger: they have three channels reaching/leaving the center, and three branching levels (i.e., 24 channels on the periphery of the circular area). On the hot side, fluid is pumped from the center to the periphery. On the cold side, fluid is pumped from the periphery to the center, and leaves the heat exchanger as a single stream. Two experimental apparatuses were built and tested. In the first design, the body of the heat exchanger was made out of plexiglass and a peripheral plenum was used to collect or distribute the working fluid to the tree structure. The measurements showed that the use of a plenum generates undesirable volumetric flow asymmetries. These lessons led to a second design, which has two major improvements: (i) the heat exchanger core was made out of aluminum and (ii) individual ports (inlets/outlets) were used for each of the peripheral channels. The hydraulic results show a relation between the appearance of volumetric flow rate asymmetries and the bifurcation angles throughout the dendritic structure. The heat transfer results are also discussed.

Effect of the Plate Thermal Resistance on the Heat Transfer Performance of a Corrugated Thin Plate Heat Exchanger

Two-dimensional conjugate conduction/convection numerical simulations were carried out for flow and thermal fields in a unit model of a counter flow type corrugated thin plates heat exchanger core. The effects of the thermal resistace of the solid plate, namely the variation of the plate thickness and the difference of the plate material, on the heat exchanger performance were examined in the Reynolds number range of 100<Re<400. Higher temperature effectiveness was obtained for a thicker plate at any Reynolds number, which was a unique feature of corrugated thin plate geometry. Detailed discussions on the thermal fields revealed that restricting the heat conduction along the plate by making the plate thinner or choosing a low thermal conductivity material causes a larger plate temperature variation along the plate, and, consequently, a smaller amount of thermal energy exchanged between two fluids.

Thermal Transport in Graphitic Carbon Foams

A number of carbon foam products are being developed for use as insulator, heat spreaders, and compact heat exchanger cores. Such foams have voids that are typically of the order of 100 μm, and the pore walls are about 10 μm. The carbon in the foam may be graphitized by heat treatment during processing. The arrangement of graphene planes in graphitic foams is a function of position within the structure of the foam. Therefore, the thermal transport process is highly dependent on the microstructure. This results in bulk conductivities that range from 1 to 200W/mK. The bulk properties of such a porous medium are difficult to determine analytically, particularly in the case of high concentration of nonspherical pores, or when the porous material is anisotropic or nonhomogeneous. A finite element analysis has been developed to calculate the bulk thermal conductivity of carbon foams with anisotropic microstructure. The effective thermal conductivity is then evaluated by comparing the results of the analytical and numerical models.

Finite Element Model of Thermal Transport in Carbon Foams

A number of carbon foam products are being developed for use as insulation, heat spreaders, and compact heat exchanger cores. The bulk properties of such a porous medium are difficult to determine analytically, particularly for the case of high concentration of non-spherical pores, or when the porous material is anisotropic or non-homogeneous. Models that predict thermal conductivity of foams often use an empirical parameter to account for the effect of pore shape and material microstructure on the conduction process. A finite element analysis has been developed to calculate the thermal conductivity of carbon foams containing micropores of different shapes. The effective thermal conductivity is then evaluated by comparing the results of the analytical and numerical models.

Analysis of thermal conduction in carbon foams

Carbon foams are a new class of porous organic materials that are being developed for use as insulation, heat spreaders, and compact heat exchanger cores. The thermal conductivity of carbon foams is difficult to determine analytically, particularly for the case of high porosity, graphitic foams that show moderate to high bulk conductivity. Models that predict thermal conductivity of foams often use an empirical parameter to account for the effect of pore shape and material microstructure on the conduction process. A finite element (FEM) analysis has been developed to calculate the thermal conductivity of carbon foams for the complete range of porosity. The results are interpreted in terms of analytical and semi-empirical solutions. It is shown that the FEM solution matches the theoretical solution that is valid for low porosity, but differs significantly from semi-empirical solutions of high porosity foams.

Boiling refrigerant‐type panel cooler (refrigerant circulation and cooling performance)

AbstractWe have developed a new compact boiling refrigerant type panel cooler. It is a closed two‐phase loop thermosiphon constructed with heat exchanger cores for an automobile air conditioning system. We confirmed higher performance of the natural refrigerant circulation type and tried to optimize the size of the refrigerant path in the boiling core by observing the internal refrigerant flow and successfully achieved much higher cooling performance. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 94–105, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20003

A heat exchanger model that includes axial conduction, parasitic heat loads, and property variations

High performance heat exchangers are a critical component in many cryogenic systems and the performance of these devices is typically very sensitive to axial conduction, property variations, and parasitic heat losses to the environment. This paper presents a numerical model of a heat exchanger in which these effects are explicitly modeled. The governing equations are derived, nondimensionalized, discretized, and solved on an exponentially distributed grid. The resulting numerical model is simple to implement and computationally efficient and can therefore easily be integrated into complex system models. The numerical model is validated against analytical solutions in the appropriate limits and then used to investigate the effect of heat exchanger end conditions (adiabatic vs fixed temperature) and radiation parasitics. The numerical model, which explicitly considers the combined effect of several loss mechanisms as they interact, is compared to simple models that consider these effects separately. Finally, the model is applied to an example heat exchanger core under a specific set of operating conditions in order to demonstrate its utility. This numerical model may also be used to examine the effect of property variations including temperature driven changes in specific heat capacity, metal conductivity, parasitic heat load, and heat transfer coefficients and is therefore useful in the design of a variety of cryogenic system components including counter- and parallel-flow heat exchangers for gas liquefaction, mixed-gas refrigeration, and reverse Brayton systems.

Fin–tube junction effects on flow and heat transfer in flat tube multilouvered heat exchangers

Three-dimensional simulations of four louver–tube junction geometries are performed to investigate the effect on louver and tube friction and heat transfer characteristics. Three Reynolds numbers, 300, 600 and 1100, based on bulk velocity and louver pitch are calculated. Strong three-dimensionality exists in the flow structure in the region where the angled louver transitions to a flat landing adjoining the tube surface, whereas the flow on the angled louver far from the tube surface is nominally two-dimensional. Due to the small spatial extent of the transition region, its overall impact on louver heat transfer is limited, but the strong unsteady flow acceleration on the top louver surface augments the heat transfer coefficient on the tube surface by over 100%. In spite of the augmentation, the presence of the tube lowers the overall Nusselt number of the heat exchanger between 25% and 30%. Comparisons with correlations derived from experiments on full heat exchanger cores show that computational modeling of a small subsystem can be used reliably to extract performance data for the full heat exchanger.

Air-cooled heat exchanger inlet flow losses

The primary purpose of this experimental investigation is to determine the influence that differences in air-cooled heat exchanger geometry have on heat exchanger inlet air flow losses. Heat exchanger geometry would not only include details pertaining to the heat exchanger finned tubes, but also the orientation of the heat exchanger finned surfaces with respect to the inlet air flow as well as to the inlet air angle of incidence on the heat exchanger. It is found that the inlet air flow losses are independent of the average air velocity through the heat exchanger. Inlet air flow losses increase with a decrease in the inlet air angle of incidence. The orientation of the heat exchanger finned surfaces has an effect on the inlet air flow losses and is primarily a function of the tube cross-sectional profile of the finned tubes. An equation based on the experimental results is formulated to calculate the heat exchanger inlet air flow losses. It is also shown that the geometric details of the heat exchanger finned tubes or heat exchanger core used in the construction of the heat exchanger could have a pronounced effect on the inlet air flow losses.

Thermodynamic Optimization of Flow Geometry in Mechanical and Civil Engineering

Recent developments in thermodynamic optimization are reviewed by focusing on the generation of optimal geometric form (shape, structure, topology) in flow systems. The flow configuration is free to vary. The principle that generates geometric form is the pursuit of maximum global performance (e.g., minimum flow resistance, minimum irreversibility) subject to global finiteness constraints (volume, weight, time). The resulting structures constructed in this manner have been named constructal designs. The thought that the same objective and constraints principle accounts for the optimally shaped flow paths that occur in natural systems (animate and inanimate) has been named constructal theory. Examples of large classes of applications are drawn from various sectors of mechanical and civil engineering: the distribution of heat transfer area in power plants, optimal sizing and shaping of flow channels and fins, optimal aspect ratios of heat exchanger core structures, aerodynamic and hydrodynamic shapes, tree-shaped assemblies of convective fins, tree-shaped networks for fluid flow and other currents, optimal configurations for streams that undergo bifurcation or pairing, insulated pipe networks for the distribution of hot water and exergy over a fixed territory, and distribution networks for virtually everything that moves in society (goods, currency, information). The principle-based generation of flow geometry unites the thermodynamic optimization developments known in mechanical engineering with lesser known applications in civil engineering and social organization. This review extends thermodynamics, because it shows how thermodynamic principles of design optimization account for the development of optimal configurations in civil engineering and social organization.

Thermodynamic optimization of heat-transfer equipment configuration in an environmental control system

In this paper, we show that many features of a heat transfer installation can be deduced from the maximization of the global performance of the greater system that employs the installation. The heat transfer installation is a series of two cross-flow heat exchangers. The greater system is the environmental control system (ECS) of an aircraft. The global performance objective is the minimization of the total thermodynamic irreversibility of the ECS. Several architectural features are deduced from principle: the relative position of the two heat exchangers, their relative sizes, and all the geometric aspect ratios of the two heat exchanger cores. We find that the optimized architecture is insensitive (robust) to changes in some of the external parameters. Robustness is a useful feature because it simplifies the design work. Furthermore, one design that is built can be expected to function at near-optimal levels when the external parameters change. The application of this method of topology optimization to more complex systems is discussed. Copyright © 2001 John Wiley & Sons, Ltd.