Complex Flow Arrangements Research Articles

A new modeling and analysis method of the indirect evaporative heat exchanger based on the heat current perspective

Indirect evaporative cooling technology has increasing importance and significance because of its high cooling efficiency and environment-friendly feature. The paper introduces a new parameter named moisture-temperature ratio that unifies the temperature difference and humidity difference and then proposes the heat current model to reveal the coupled heat and mass transfer process. For an indirect evaporative heat exchanger, the driven potential of the coupled heat and mass transfer processes is considered and can be modified as the difference between the inlet temperature of the hot fluid and the inlet wet-bulb temperature of the cold wet air. On this basis, we derive and obtain the thermal resistance expressions of parallel-flow, counter-flow and other complex flow arrangements indirect evaporative heat exchangers. Finally, the comparison with the experimental data shows that the discrepancy of the outlet temperature of the hot air obtained by the heat current model is less than less than 0.85 °C. Besides, the proposed heat current model is feasible and convenient for analyzing the influences of inlet humidity, inlet temperature, and channel height on the overall performance of heat exchanger, and provides an effective solution for the indirect heat exchanger design and performance analysis.

In-Situ Measurements in Microscale Gas Flows-Conventional Sensors or Something Else?

Within the last few decades miniaturization has a driving force in almost all areas of technology, leading to a tremendous intensification of systems and processes. Information technology provides now data density several orders of magnitude higher than a few years ago, and the smartphone technology includes, as well the simple ability to communicate with others, features like internet, video and music streaming, but also implementation of the global positioning system, environment sensors or measurement systems for individual health. So-called wearables are everywhere, from the physio-parameter sensing wrist smart watch up to the measurement of heart rates by underwear. This trend holds also for gas flow applications, where complex flow arrangements and measurement systems formerly designed for a macro scale have been transferred into miniaturized versions. Thus, those systems took advantage of the increased surface to volume ratio as well as of the improved heat and mass transfer behavior of miniaturized equipment. In accordance, disadvantages like gas flow mal-distribution on parallelized mini- or micro tubes or channels as well as increased pressure losses due to the minimized hydraulic diameters and an increased roughness-to-dimension ratio have to be taken into account. Furthermore, major problems are arising for measurement and control to be implemented for in-situ and/or in-operando measurements. Currently, correlated measurements are widely discussed to obtain a more comprehensive view to a process by using a broad variety of measurement techniques complementing each other. Techniques for correlated measurements may include commonly used techniques like thermocouples or pressure sensors as well as more complex systems like gas chromatography, mass spectrometry, infrared or ultraviolet spectroscopy and many others. Some of these techniques can be miniaturized, some of them cannot yet. Those should, nevertheless, be able to conduct measurements at the same location and the same time, preferably in-situ and in-operando. Therefore, combinations of measurement instruments might be necessary, which will provide complementary techniques for accessing local process information. A recently more intensively discussed additional possibility is the application of nuclear magnetic resonance (NMR) systems, which might be useful in combination with other, more conventional measurement techniques. NMR is currently undergoing a tremendous change from large-scale to benchtop measurement systems, and it will most likely be further miniaturized. NMR allows a multitude of different measurements, which are normally covered by several instruments. Additionally, NMR can be combined very well with other measurement equipment to perform correlative in-situ and in-operando measurements. Such combinations of several instruments would allow us to retrieve an “information cloud” of a process. This paper will present a view of some common measurement techniques and the difficulties of applying them on one hand in a miniaturized scale, and on the other hand in a correlative mode. Basic suggestions to achieve the above-mentioned objective by a combination of different methods including NMR will be given.

Modelling of a Superheater Heat Exchanger with Complex Flow Arrangement Including Flow and Temperature Maldistribution

ABSTRACTA network approach was used to develop a thermofluid process model of a cross-flow primary superheater heat exchanger with complex flow arrangement in the convective pass of a coal-fired boiler. The model solves the transient one-dimensional forms of the conservation equations for mass, energy, and momentum, combined with the applicable closure relations, boundary values, and initial values. The dual-tube 12-pass superheater was discretized along the flue gas flow path as well as along the steam flow path. The model accounts for the convective thermal resistance on the steam side, the conductive thermal resistances of the tube wall, and scaling or fouling on the tube walls, as well as the convective and radiative thermal resistances on the flue gas side. The model was qualitatively validated using real plant data and for reference purposes also systematically compared to conventional lumped parameter models. The ability of the model to analyze the effect of the ramp rate during load changes on the tube metal temperature was demonstrated, as well as the ability to determine the maldistribution of flow and temperature on the steam and flue gas sides. Models such as this can be employed to study complex thermofluid process phenomena that may occur during intermittent, transient, and low-load operation of power plants. This could form the basis for improving operations and for the development of advanced tools for online process condition monitoring.

Mathematical Modeling of Cross-Flow Tube Heat Exchangers With a Complex Flow Arrangement

The general principles of mathematical modeling of heat transfer in cross-flow tube heat exchangers with complex flow arrangements that allow the simulation of multipass heat exchangers with many tube rows are presented. The finite-volume method is used to solve the system of differential equations for temperature of the both fluids and the tube wall with appropriate boundary conditions. A numerical model of a multipass steam superheater with 12 passes is presented. The convection and radiation heat transfer on the flue gas side are accounted for. In addition, the deposit layer is assumed to cover the outer surface of the tubes. Comparing the computed and measured steam temperature increase over the entire superheater allows for determining the thermal resistance of the deposits layer on the outer surface of the superheater. The developed modeling technique can especially be used for modeling tube heat exchangers when detailed information on the tube wall temperature distribution is needed.

Effect of scale deposits on the internal surfaces of the tubes on the superheater operation

Abstract A mathematical model of the steam superheater exchanger with distributed parameters has been developed. Scale deposits were assumed to be present on the internal tube surfaces. It was assumed that the inner tube surfaces are covered by a thin layer of scale deposits. The finite volume method was used to solve partial differential equations describing flue gas, tube wall and steam temperature. The developed modeling technique can especially be used for modeling tube heat exchangers when detail information on the tube wall temperature distribution is needed. The numerical model of the superheater developed in the paper can be used for modeling of the superheaters with complex flow arrangement accounting scales on the internal tube surfaces. Using the model proposed the detailed steam, wall and flue gas temperature distribution over the entire superheater can be determined. The steam pressure distribution along its path flow and the total heat transfer rate can also be obtained. The calculations showed that the presence of scale on the internal surfaces of the tubes cause the steam temperature decrease and the heat flow rate transferred from the flue gas to the steam. Scale deposits on the inner surfaces of the tubes cause the tube wall temperature growth and can lead to premature wear of tubes due to overheating.

Mathematical modelling of tube heat exchangers with complex flow arrangement

Mathematical modelling of tube heat exchangers with complex flow arrangement General principles of mathematical modelling of transient heat transfer in cross-flow tube heat exchangers with complex flow arrangements which allow a simulation of multipass heat exchangers with many tube rows are presented. First, a system of differential equations for the transient temperature of both fluids and the tube wall with appropriate boundary and initial conditions is formulated. Two methods for modelling heat exchangers are developed using the finite difference method and finite volume method. A numerical model of multipass steam superheater with twelve passes is presented. The calculation results are compared with the experimental data.

Effectiveness - NTU data and analysis for air conditioning and refrigeration air coils

A simulation program based on a control volume analysis has been used in the evaluation of the (e, NTU) relationship for coils of complex geometry and flow arrangement. The simulation program has been evaluated through simple geometry and flow arrangement coils. The program results compare very well with correlations for simple cross flow coils, and a number of rows up to four. It has also been determined that closed form correlations developed for coils of an infinite number of tube rows are inadequate for those with number of rows in the range between 5 and 10. In addition, it has been found that closed form (e, NTU) correlations for cross flow coils with the same tube arrangement and number of rows might lead to inaccuracies higher than 10% in the evaluation of the effectiveness of coils of complex flow arrangement.

Effectiveness-ntu computation with a mathematical model for cross-flow heat exchangers

Due to the wide range of design possibilities, simple manufactured, low maintenance and low cost, cross-flow heat exchangers are extensively used in the petroleum, petrochemical, air conditioning, food storage, and others industries. In this paper a mathematical model for cross-flow heat exchangers with complex flow arrangements for determining epsilon -NTU relations is presented. The model is based on the tube element approach, according to which the heat exchanger outlet temperatures are obtained by discretizing the coil along the tube fluid path. In each cross section of the element, tube-side fluid temperature is assumed to be constant because the heat capacity rate ratio C*=Cmin/Cmax tends toward zero in the element. Thus temperature is controlled by effectiveness of a local element corresponding to an evaporator or a condenser-type element. The model is validated through comparison with theoretical algebraic relations for single-pass cross-flow arrangements with one or more rows. Very small relative errors are obtained showing the accuracy of the present model. epsilon -NTU curves for several complex circuit arrangements are presented. The model developed represents a useful research tool for theoretical and experimental studies on heat exchangers performance.

Alternative Design Approach for Multipass and Multi-Stream Plate Heat Exchangers for Use in Heat Recovery Systems

This work presents an alternative approach for the preliminary design of plate and frame heat exchangers in complex flow arrangements and looks at the use of this type of exchanger in multi-stream applications. The need for a new design approach arises from the fact that most current design algorithms are industrially owned and thus are not readily available to the practicing engineer. The essential information required for the design of plate and frame heat exchangers are the heat transfer and friction performance data, which again are industrially owned. However, a few correlations for the most common type of plates have been reported in the open literature and, in this work, are used for the development of a design approach that exploits the concept of full utilization of allowable pressure drop. This method can be used as a first estimate for the determination of the surface area requirements of single-phase, two-stream heat exchangers. It is shown how the approach can be extended to the targeting and design of heat recovery systems or multi-stream exchangers where plate heat exchangers are a suitable option.

Advances in Science and Technology of Compact Heat Exchangers

Significant advances have taken place in the theory, analysis, design and optimization, manufacturing, and technology of compact heat exchangers (CHEs) over the last 20–25 years. The objective of this D. Q. Kern Award paper is to summarize and assess (where appropriate) these advances in CHEs related to the following specific areas of CHEs: (1) advances in two-fluid exchanger effectiveness and NTU results for highly complex flow arrangements, (2) heat transfer and pressure drop analyses, (3) the role of CFD in the design and analysis of header and manifold design, (4) recuperator design procedure, (5) design data for compact heat exchangers, (6) thermodynamic modeling and analysis, (7) brazing of compact heat exchangers, and (8) advancement in CHEs for new applications such as fuel cells and microturbines. Some challenges and opportunities will be summarized to advance the science and technology further.

Exchanger Performance Behavior Through Irreversibility Analysis for 1-2 TEMA G Heat Exchangers

Abstract The objective of this paper is to illustrate, discuss, and explain the interrelationship between the temperature difference irreversibility and heat exchanger effectiveness to clarify the performance trends of exchangers with some complex flow arrangements. This is because there is no physical explanation provided for the following results presented by Shah and Skiepko (ASME J. Heat Transfer, 126, pp. 994–1002, 2004): the heat exchanger effectiveness can be maximum, having an intermediate value or minimum at the maximum irreversibility operating point depending upon the flow arrangement of two fluids; similarly, the heat exchanger effectiveness can be minimum or maximum at the minimum irreversibility operating point. The analysis of such complex performance behavior is presented in this paper with an example of overall parallelflow and counterflow 1-2 TEMA G exchangers. This is accomplished by the decomposition of complex flow arrangements into simple subexchangers, and then the overall irreversibility trends for the exchangers are explained by irreversibilities produced due to temperature difference and fluid mixing in component subexchangers. It is shown for 1-2 TEMA G exchangers that the temperature difference irreversibility for a pure parallelflow subexchanger passes through a maximum at finite value of NTU1, and then approaches 0 when NTU1→∞. On the contrary, the irreversibility for a pure counterflow subexchanger attains a minimum value at finite NTU1 and then increases with NTU1 and approaches maximum at NTU1→∞ for 1–2 TEMA G exchangers. This is because the temperatures at the inlet of the subexchangers are variable and dependent on the exit temperatures from the preceding subexchangers. Detailed exchanger effectivenesses and temperature ratios are presented as a function of NTU1 for the explanation.

A new approach for thermal performance calculation of cross-flow heat exchangers

A new numerical methodology for thermal performance calculation in cross-flow heat exchangers is developed. Effectiveness–number of transfer units ( ε–NTU) data for several standard and complex flow arrangements are obtained using this methodology. The results are validated through comparison with analytical solutions for one-pass cross-flow heat exchangers with one to four rows and with approximate series solution for an unmixed–unmixed heat exchanger, obtaining in all cases very small errors. New effectiveness data for some complex configurations are provided.

Entropy Generation Extrema and Their Relationship With Heat Exchanger Effectiveness—Number of Transfer Unit Behavior for Complex Flow Arrangements

While the concept of minimum irreversibility is associated with the maximum energy efficiency for energy conversion processes in thermal systems, we have found that it is not quite applicable to the heat exchanger analysis. We have shown that the heat exchanger effectiveness can be maximum, having an intermediate value or minimum at the maximum irreversibility operating point depending on the flow arrangement of the two fluids. Similarly, the heat exchanger effectiveness can be minimum or maximum at the minimum irreversibility operating point. The objective of this paper is to illustrate and discuss such heat exchanger performance and irreversibility trends by combining the temperature difference irreversibility with the P-NTU results for complex flow arrangements.

Dynamic modelling and simulation of plate heat exchangers under milk fouling

This work presents the mathematical modelling and simulation of complex plate heat exchanger arrangements under milk fouling, using detailed dynamic models. A complex fouling model based on a reaction/mass transfer scheme is coupled with a general thermal dynamic model of plate heat exchangers. All the important factors affecting milk heat treatment are formally quantified. The final model comprises a set of partial differential, integral and algebraic equations. Parameter estimation analysis is performed based on the solution of a dynamic optimization problem. The simulation results are in a good agreement with available experimental work. Three different configurations with complex flow arrangements are considered to illustrate aspects of fouling behaviour. The simulation results provide significant insight into the key factors affecting milk fouling.

A Review of Solution Methods for Determining Effectiveness-NTU Relationships for Heat Exchangers With Complex Flow Arrangements

A number of approaches have been employed for the design and analysis of two-fluid heat exchangers. These approaches include the effectiveness (ε or P)-number of transfer units (NTU) method, the mean temperature difference method, and the ones using the Mueller and Roetzel charts. A comprehensive review is presented of solution methods for obtaining effectiveness-NTU relationships for simple and complex heat exchangers and heat exchanger assemblies. These include analytical methods for obtaining exact solutions, approximate (including semi-numerical and analog) methods, numerical methods, matrix formalism, and additional methods based on the properties of the exchanger configurations. Additional details that pertain to the Laplace transforms and matrix formalism that includes the chain rule methodology are discussed. A step-by-step procedure that illustrates the chain rule methodology is presented. This review article includes 73 references.

Modelling and simulation of complex plate heat exchanger arrangements under milk fouling

This paper presents the detailed mathematical modelling and simulation of complex plate heat exchangers under milk fouling, using computational fluid dynamics models. A complex fouling model based on a reaction/mass transfer scheme is described where all the important factors during milk heat treatment are quantified in a formal way. This model is coupled with detailed dynamic models of plate heat exchangers (PHEs). The final model comprises a set of partial differential, integral and algebraic equations. Parameter estimation analysis is performed based on the solution of a dynamic optimization problem. The simulation results have been compared with available experimental work and a satisfactory agreement has been found. Three different industrial configurations with complex flow arrangement are considered to illustrate aspects of fouling behaviour.

Characteristics of a Power Station Boiler

An experimental investigation of the flow patterns in a model of a power station boiler has been carried out under isothermal conditions, to produce data for computational fluid dynamic code validation. Measurements were made in three dimensions on a single swirl burner in isolation, in two dimensions on a bank of 12 burners in a 4 × 3 grid and in the model boiler with this bank installed. The single burner exhibited a central reverse flow zone with high levels of turbulence at its boundaries. The general flow pattern found in the single burner was greatly distorted when interaction with surrounding burners in the bank occurred. Several unexpected secondary flows occurred. The model boiler displayed a complex flow arrangement with recirculation zones and two stagnation points.

Effectiveness-number of transfer units relationships for heat exchanger complex flow arrangements

Heat exchanger performance (or terminal temperatures), size and fluid flow rates are related in a dimensionless form in terms of the exchanger effectiveness, number of transfer units and heat capacity rate ratio. Such relationships are essential for design and analysis of any heat exchangers. Over the last 100 years, many heat exchanger flow arrangements have been analyzed and reported in the literature. However, since 1969, several very powerful methods have been developed to analyze complicated flow arrangements for two-fluid recuperators. These are matrix formalism, chain rule, and rules for exchangers with one fluid mixed, among others. These methods are briefly summarized in the paper with illustrative examples. Using these methods, 18 new recuperator flow arrangements have been analyzed and the results are presented in closed-form formulas assuming constant overall heat transfer coefficient and fluid properties. The results summarized here together with those published in the open literature should then provide the reader with an idea as to where to concentrate future research efforts on the subject.

Finite element analysis of shell and tube heat exchanger

A finite element model to predict temperature distribution in heat exchangers is reported. The model can be effectively used to analyse and design the heat exchangers with complex flow arrangements for which no regular design procedure is available. Illustrations are provided to explain the application of the method for the analysis of shell and tube heat exchangers.