Heat Transfer Area Distribution Research Articles

Optimization of solution circulation rate and heat transfer area distribution for hot-water driven LiBr/H2O absorption chillers

Optimization of solution circulation rate and heat transfer area distribution for hot-water driven LiBr/H2O absorption chillers

Effect of heat transfer area distribution on frosting performance of refrigerator evaporator

The present study investigates the effect of the distribution of available heat transfer area on the thermohydraulic behavior of a typical fin-and-tube type evaporator used in a domestic frost-free refrigerator. Previous studies in the literature have not explored the effect of the number of fin rows and fin pitch distribution across the fin rows on the performance of fan-supplied evaporator geometries under frosting. The present study attempts to address this by developing a thermohydraulic model of the evaporator to predict the heat transfer rate, pressure drop and frost formation. A distributed evaporator model is developed by dividing the evaporator into individual control volumes and the balance equations are applied along with a suitable semi-empirical frost growth model to predict the rate of frost formation. Experiments were performed in a purpose-built test facility to validate the numerical model. The developed model is employed to compare different heat exchanger geometries with a constant area but varying in total fin rows and fin pitch distribution. Results show that distributing the available heat transfer area among a higher number of fin rows results in longer successive defrost intervals. However, the evaporator occupies more volume. For a given heat transfer area and fin rows, longer operating time can be achieved by selecting fin pitches that provide uniform blockage ratio across the fin rows. It is observed that for a specific fin pitch at the leading row, reducing the fin pitch linearly across the fin rows resulted in uniform blockage ratio across the fin rows.

Эльбель С., Хрняк П. Оптимизация рабочих режимов транскритической углекислотной холодильной установки с двухступенчатым компрессором = Elbel S., Hrnjak P. Performance optimization of two-stage compressor system using transcritical R744 / пер. с англ. М. А. Федоровой

The use of transcritical R744 systems has become increasingly popular in recent years in a variety of different applications. For applications that span a wide temperature range between the heat source and heat sink, the use of two-stage compressor results in numerous advantages in terms of efficiency and compressor discharge temperature. This paper presents experimental data for a transcritical R744 compressor system operating at high heat rejection temperatures. A comprehensive system model was developed and validated with the experimental results. Based on this, the simulation tool was used to further optimize the system design specifically to accommodate the two-stage compression process. The optimum heat transfer area distribution has been determined to simultaneously ensure efficient intercooling at intermediate pressure and gas cooling at the high-pressure level. Simultaneously, the system was also optimized with respect to optimal intermediate pressure and the results show that for this particular system, the optimum intercooler pressure deviated substantially from the standard design approach that uses the geometric mean between suction and discharge pressures

Thermal Optimization Design of Heat Exchanger in Supersonic Engine with Parameters’ Fluctuation

Abstract Precooled engine is a highly expected solution to achieve supersonic transport. As the crucial component, heat exchangers protect other components from ultrahigh temperature. In traditional design methods, the nominal result is multiplied by a safety factor, whose selection entirely depends on experience, ensuring sufficient working margin to cope with fluctuation of parameters. For aero-engine, heat exchangers must work reliably with minimum weight. An advanced method of thermal optimization design with parameters’ fluctuation is proposed and proved to be effective by experimental verification. The heat transfer area can be quantitatively linked with the design confidence level, considering the coupling effect of various parameters’ fluctuation. The probability density distribution of heat transfer area has the characteristic of positive skewness distribution. With the increase of design confidence, the required heat transfer area is growing faster and faster. After optimization, the design of heat exchanger meets the requirements and the weight is effectively controlled.

Performance optimization of two-stage compressor system using transcritical R744

The use of transcritical R744 systems has become increasingly popular in recent years in a variety of different applications. For applications that span a wide temperature range between the heat source and heat sink, the use of two-stage compressor results in numerous advantages in terms of efficiency and compressor discharge temperature. This paper presents experimental data for a transcritical R744 compressor system operating at high heat rejection temperatures. A comprehensive system model was developed and validated with the experimental results. Based on this, the simulation tool was used to further optimize the system design specifically to accommodate the two-stage compression process. The optimum heat transfer area distribution has been determined to simultaneously ensure efficient intercooling at intermediate pressure and gas cooling at the high-pressure level. Simultaneously, the system was also optimized with respect to optimal intermediate pressure and the results show that for this particular system, the optimum intercooler pressure deviated substantially from the standard design approach that uses the geometric mean between suction and discharge pressures.

Analysis of the heat transfer area distribution in a frosted plain fin-and-tube geometry

The development of frost is a phenomenon that deteriorates thermohydraulic performance on heat exchangers. In this study, several heat-transfer area distributions on a fin-and-tube geometry are proposed and their performances as frost develops are compared using simulation. The frost development at a specific location is determined using a segment analysis. In each segment, a semiempirical model to predict the frost growth based on air temperature, velocity, relative humidity and surface temperature is applied. The analysis considers airflow redistribution among channels, leading to changes in heat transfer and frosting rates with time. Results show that a geometry that allows even flow distribution along the operation time is less sensitive to thermohydraulic deterioration. An area distribution with larger fin spacing and fin length presents an advantage, particularly on the pressure drop, which allows longer operation time between defrost cycles.

Performance investigation and exergy analysis of two-stage desiccant wheel systems

Two-stage desiccant wheel systems are an effective way to improve the dehumidification performance. In the present study, the performances of a one-stage system and a two-stage system with identical heat transfer areas are compared, with particular emphasis on the required heating source temperature. The exergy and unmatched coefficient (ξ) are applied to analyze the destruction of the heat and mass transfer processes. Compared to the one-stage system, the regeneration temperature (Tr) of the two-stage system is lower. The required hot water temperature (Th) depends on the supplied water flow rate and Ap/Ar of the desiccant wheel. When Ap/Ar = 1, Th of the two-stage system is lower only when the supplied water flow rate is relatively high. And due to different heat transfer area distribution demand, the exergy destruction of two-stage system is higher than one-stage system. When Ap/Ar = 3, the two-stage system has greater advantages. ξ of the desiccant wheel decreases from 2.9 to 2.2 when the number of stages increases from 1 to 2, leading to lower exergy destruction of the desiccant wheels and higher exergy efficiency.

Modeling, simulation and optimization of a vapor compression refrigeration system dynamic and steady state response

This paper introduces a dimensionless simplified mathematical model of a vapor compression refrigeration (VCR) system, in order to optimize the system dynamic response. The model combines principles of thermodynamics, heat and mass transfer applied to the system components with empirical correlations, assigning thermodynamic control volumes to each component, which yield a system of ordinary differential equations with respect to time that is integrated explicitly and accurately with low computational time. Appropriate dimensionless groups are identified, and the results are presented in the form of normalized charts for general application to similar systems. Experiments were conducted using an industrial chiller that was instrumented for real time data acquisition in order to determine model adjustment parameters through the solution of the inverse problem of parameters estimation (IPPE) for a 300W thermal load. The model was then experimentally validated using the adjusted parameters by direct comparison of simulation results to the system measured thermal response for a 1200W thermal load with good agreement. After experimental validation, the study addressed the optimization of the heat exchangers heat transfer area inventory for minimum pull down time. A system performance comparison is conducted for three refrigerant fluids: (i) a banned refrigerant (R12); (ii) its original ozone depletion harmless substitute, but with a high global warming potential, GWP (R134a), and (iii) one of the current substitutes of R134a, i.e., R1234yf. The dynamic results show that, for a system originally designed for R12, substitution with R1234yf depicts a closer performance to R12 than R134a. The optimal configuration that leads to steady state maximum system first (or coefficient of performance, COP), and second law efficiencies is also pursued. The normalized results for refrigerants R12, R134a, and R1234yf show that an optimal heat transfer area distribution in both evaporator and condenser, represented by an evaporator to total system heat exchanger area ratio x4,opt≅0.55, leads the system to minimum pull down time and maximum system 2nd law efficiency, whereas x4,opt≅0.4 to maximum COP, when the heat exchangers global heat transfer coefficients are of the same magnitude. The difference in the optimum location when the objective function is the pull-down time (and second law efficiency) and COP is due to the fact that the first law analysis does not fully capture the thermodynamic losses due to changes in heat exchangers’ areas, which stresses the importance of a system second law assessment for realistic results. However, changes in the optima location are observed when the ratio of heat exchangers global heat transfer coefficients departs from 1. The obtained maxima and minima are sharp in the searching interval (0.1⩽x4⩽0.9), showing up to a 189.6% and 93.37% variation, fmax-fminfmax/min×100, in which f is either the calculated system pull down time or second law efficiency, respectively, which was observed with refrigerant R1234yf, which points out their importance for actual HVAC–R (heating, ventilation, air conditioning and refrigeration) vapor compression systems design.

Staging of the Fischer–Tropsch reactor with an iron based catalyst

The Fischer–Tropsch reactor is sectioned into stages based on the systematic method given by Hillestad (2010). The design functions are optimized to maximize the concentration of C11+ at the end of reactor path. The decision variables are fluid mixing, hydrogen distribution, heat transfer area distribution, coolant temperature, and catalyst concentration. With the path temperature constrained by 250°C, staging of the reactor will increase the concentration of C11+. For a three-stage reactor, the concentration is increased by 2.50% compared to a single-stage reactor. The optimal mixing structure is plug flow to have the maximum possible conversion. A case study is conducted to separate and distribute hydrogen along the reactor path. This will reduce H2/CO at the beginning of the path and increase chain growth probability. The results show that for a three-stage reactor, the concentration of C11+ is increased by 15.93% compared to single-stage reactor.

Systematic staging in chemical reactor design

Abstract The foundation and implementation of a method for systematic reactor design is described. The reactor path is sectioned into stages where each stage is designed so as to optimize an overall objective. This is a further development of a previously proposed method for designing chemical reactors ( Hillestad, 2004, 2005 ). Reactants pass through a series basic operations or functions to form the desired products. The basic operations are represented by design functions on the path volume. The design functions are fluid mixing (dispersion), distribution of extra feed points, distribution of heat transfer area and coolant temperature, catalyst dilution distribution and more. The conceptual reactor design problem is solved as an optimal control problem. A direct method is applied where both the design functions and the state variables are discretized. The realization of the optimization is a staged process string of multi-functional units. A kinetic model of the gas phase methanol synthesis is used as an example. By applying the method on the model, a staged reactor design with less heat transfer area and higher production is possible.

Toyo Engineering in alliance to commercialize small GTL plants using micro-channel reactors

The Fischer–Tropsch reactor is sectioned into stages based on the systematic method given by Hillestad (2010). The design functions are optimized to maximize the concentration of C11+ at the end of reactor path. The decision variables are fluid mixing, hydrogen distribution, heat transfer area distribution, coolant temperature, and catalyst concentration. With the path temperature constrained by 250 °C, staging of the reactor will increase the concentration of C11+. For a three-stage reactor, the concentration is increased by 2.50% compared to a single-stage reactor. The optimal mixing structure is plug flow to have the maximum possible conversion. A case study is conducted to separate and distribute hydrogen along the reactor path. This will reduce H2/CO at the beginning of the path and increase chain growth probability. The results show that for a three-stage reactor, the concentration of C11+ is increased by 15.93% compared to single-stage reactor.

A systematic generation of reactor designs: II. Non-isothermal conditions

A new method is proposed for systematic generation of conceptual design of reactor networks. Here, non-isothermal conditions are considered. Optimal distribution of heat transfer area, in addition to optimal mixing and distribution of feeds are targeted. Given temperature and composition of feed streams, the objective is to find the sequence and size of ideal reactors, the distribution of extra feed streams, the heat transfer distribution along the reactor path and the total reaction time that maximizes the space–time yield of the key product component. The method can be applied to solve problems of any number of components and reactions. The method is applied to industrially important processes such as the methanol synthesis and the steam methane reforming process. Interesting results are obtained with respect to the required heat exchange area.

Theoretical analysis and experimental confirmation of the uniformity principle of temperature difference field in heat exchanger

Theoretical analysis and experimental confirmation for the principle to improve the thermal performance of heat exchangers is performed in this paper. The more uniform the temperature difference field (TDF), the higher the effectiveness of heat exchanger for the fixed N tu and C r. The uniformity of the TDF and the effectiveness of 13 types of heat exchangers are studied analytically and numerically, and the results support the uniformity principle of TDF. Further verification is given by the asymptotic solution for TDF in terms of a recurrence formula of heat transfer area distribution for the same kind of heat exchanger. The analysis of entropy generation caused by the heat transfer indicates that the uniformity principle of TDF satisfies the second law of thermodynamics. The results of the experimental setup, governed by the uniformity principle of TDF, show that the effectiveness increases with the increase of the uniformity of TDF. Heat exchanger effectiveness for the best flow distribution was found to be 11.2% greater than that of the conventional flow distribution without associated increase in pressure drop. Two ways, redistributing heat transfer areas and varying the connection between tubes, are presented for improving the uniformity of TDF and increasing effectiveness for the crossflow heat exchangers.

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.

Retrofit of pressure drop constrained heat exchanger networks

The study of retrofit of heat exchanger networks is usually restricted to constant heat transfer coefficients, and pressure drop constraints due to the additional exchanger area is neglected. In this work, a new approach considering the distribution of heat transfer area and pressure drop in retrofit is presented. The problem is described as a non-linear model, and the additional area required for the new network condition and available pressure drop are estimated based on economical optimisation (or process requirements).

Performance of Crossflow Heat Exchanger

In search of ways to obtain uniform outlet temperature distribution on crossflow heat exchangers, theoretical analyses by means of Laplace transformation are further developed, and experimental investigations are carried out on both the plate fin type and the fin tube type crossflow heat exchangers. The analyses and experiments are conducted under the conditions of (1) various distribution patterns of fluid flow rate and (2) various distribution patterns of heat transfer area, and the results thereof are compared. In consequence, by setting the fluid flow rate distribution, as confirmed by the analyses, uniform outlet temperature in an appreciable degree can be obtained over a wide range of NTU, and better temperature efficiency is achieved. By varying the heat transfer area distribution, similar results are obtained, but this method is impractical compared with the former.