Thermal Resistance Of Heat Exchanger Research Articles

Enhanced behaviour of a passive thermoelectric generator with phase change heat exchangers and radiative cooling

Heat exchangers are essential to optimize the efficiency of Thermoelectric Generators (TEGs), and heat pipes without fans have proven to be an advantageous design as it maintains the characteristic robustness of thermoelectricity, low maintenance and lack of moving parts. However, the efficiency of these heat exchangers decreases under natural convection conditions, reducing their heat transfer capacity and thus thermoelectric power production. This work reports on a novel heat exchanger that combines for the first time, phase change and radiative cooling in a thermoelectric generator to improve its efficiency and increase the production of electrical energy, specially under natural convection. For this, two thermoelectric generators with heat-pipes on their cold sides have been tested: one with the radiative coating and the other without it. Their thermal resistances have been determined and the electric power output was compared under different working conditions, namely, natural convection and forced convection indoors and outdoors. The experimental tests show a clear reduction of the heat exchanger thermal resistance thanks to the radiative coating and consequently, an increase of electric production 8.3 % with outdoor wind velocities of 1 m/s, and up to 54.8 % under free convection conditions. The application of the radiative surface treatment is shown to result in a more stable electrical energy production, suppressing the drastic decrease in the generated electric power that occurs in thermoelectric generators when they work under free convection.

Performance Evaluation of Thermoelectric Cooling with Two Difference Fluids Medium

Thermoelectric has been used in various applications related to cooling systems (TEC). Most researchers focused on expanding the application of TEC and improving heat transfer. The improvement of the heat transfer relied on the configuration, heat exchanger, and fluid medium. However, no previous work has reported the influence of air and water as the fluid’s medium on the TEC performance. Therefore, in this study, the performance of TEC with water and air as working fluids is evaluated experimentally. Besides, several input parameters are controlled to evaluate the TEC performance under different conditions. The results reveal that the variation of working fluid and input parameters influenced the overall TEC output. The increment of TEC cooling capacity is proportional to the input power, mass flow rate, and inlet temperature of the working fluid. While the input power and inlet temperature also vary the heat exchanger thermal resistance. The overall thermal resistance of the water block is averagely ten times lower than that of the heat sink, therefore, the water block is significantly better compared to the heat sink. While the highest COP obtained from the water and air system is 1.72 and 1.41, respectively.

A New Approach for Characterizing Pile Heat Exchangers Using Thermal Response Tests

Pile heat exchangers offer a cost effective route to implementation of ground-source heat pump systems for many large commercial buildings compared with traditional boreholes. Such projects typically use thermal response tests to determine the key input parameters for system design, namely soil thermal conductivity and heat exchanger thermal resistance. However, this brings challenges for pile heat exchanger based systems, where in situ thermal response tests are known to be less reliable due to the large thermal capacity of the pile. This paper presents a new “black box” resistance capacitive model for applications to pile thermal response tests. The approach is tested against case study data and shown to perform well. Additional test duration savings are shown to be possible if a novel combination of borehole and pile thermal response tests is applied together to determine design parameters.

Optimization of thermoelectric modules for maximum cooling capacity

A theoretical investigation to optimize thermoelectric modules for maximization of cooling capacity is performed using a novel one-dimensional analytic model, in which the thermal resistances of heat exchangers as well as the thermal and electrical contact resistances inherent in the modules are taken into account. New dimensionless parameters are introduced to avoid the difficulties in optimizing actual (dimensional) cooling capacity encountered in previous dimensional analyses. It is shown that there exist an optimum thermoelement length and an optimum electric current which maximize the cooling capacity when the heat source temperature, heat sink temperature, the thermal resistances of heat exchangers and the contact resistances are given. The effects of the heat exchangers’ thermal resistances and the contact resistances on the attainable maximum cooling capacity, COP, the optimum electric current and thermoelement length are shown.

Influence of transverse vibrations on convective heat transfer in parallel flow tube‐in‐tube heat exchanger

AbstractTube‐in‐tube heat exchangers are widely used in food processing industries and wastewater treatment for both heating and cooling. Enhancement techniques namely active, passive, and compound are developed to reduce the thermal resistance in heat exchangers by improving convective heat transfer with or without increase in surface area. The present experimental study is aimed at analyzing the influence of vibrations on the convective heat transfer of a parallel flow tube‐in‐tube heat exchanger. The heat exchanger is placed in horizontal position and is subjected to transverse vibrations under turbulent fluid flow conditions. Experiments were performed at four frequencies (20, 40, 60, and 100 Hz), three amplitudes (1, 2, and 3 m/s2), and three vibration generator positions along its length, in the Reynolds number range of 10 710 to 21 420. An enhancement in Nusselt number is found with vibration than without vibration throughout the entire range of Reynolds numbers. A maximum enhancement of 33% at 40 Hz frequency, 3 m/s2 amplitude, and vibration generator position at three‐fourth of the tube length was observed. Empirical correlations are developed for Nusselt number to determine the heat transfer coefficient with vibration with an error of 10%.

Modelling of Polymeric Shell and Tube Heat Exchangers for Low-Medium Temperature Geothermal Applications

Improvements in using geothermal sources can be attained through the installation of power plants taking advantage of low and medium enthalpy available in poorly exploited geothermal sites. Geothermal fluids at medium and low temperature could be considered to feed binary cycle power plants using organic fluids for electricity “production” or in cogeneration configuration. The improvement in the use of geothermal aquifers at low-medium enthalpy in small deep sites favours the reduction of drilling well costs, and in addition, it allows the exploitation of local resources in the energy districts. The heat exchanger evaporator enables the thermal heat exchange between the working fluid (which is commonly an organic fluid for an Organic Rankine Cycle) and the geothermal fluid (supplied by the aquifer). Thus, it has to be realised taking into account the thermodynamic proprieties and chemical composition of the geothermal field. The geothermal fluid is typically very aggressive, and it leads to the corrosion of steel traditionally used in the heat exchangers. This paper analyses the possibility of using plastic material in the constructions of the evaporator installed in an Organic Rankine Cycle plant in order to overcome the problems of corrosion and the increase of heat exchanger thermal resistance due to the fouling effect. A comparison among heat exchangers made of commonly used materials, such as carbon, steel, and titanium, with alternative polymeric materials has been carried out. This analysis has been built in a mathematical approach using the correlation referred to in the literature about heat transfer in single-phase and two-phase fluids in a tube and/or in the shell side. The outcomes provide the heat transfer area for the shell and tube heat exchanger with a fixed thermal power size. The results have demonstrated that the plastic evaporator shows an increase of 47.0% of the heat transfer area but an economic installation cost saving of 48.0% over the titanium evaporator.

Investigations into thermal resistance of tunnel lining heat exchangers

Geothermal energy is a promising and sustainable source that can reduce current dependence on conventional fuels for thermal energy production. To exploit this source of energy thermo-active geostructures such as tunnel lining heat exchangers are being investigated theoretically as well as experimentally. These geostructures are composed of concrete panels embedded with reinforcement cages fitted with absorber pipes. Several engineering projects in China, Finland and Italy have deployed such heat exchangers in tunnels. To achieve efficient energy production, characterisation of these systems require realistic models of the substructure heat exchanger. Therefore investigations into thermal resistance of the heat exchanger is vital. The present study is concerned with quantifying the thermal resistance of tunnel lining heat exchangers where the thermal boundary surfaces are applied at surfaces representing the adjacent ground and the exposed concrete, in addition to the pipe surface. Steady state temperature distribution in a two dimensional cross section of a tunnel lining heat exchanger is investigated using the boundary collocation least squares method. Design parameters including pipe and tunnel lining specifications are used as model inputs.

Detailed Model of a Thermoelectric Generator Performance

Abstract The paper deals with mathematical modeling of heat transfer phenomena occurring in a system containing thermoelectric elements. The main focus was on creating a useful computational tool for designing, validating, testing, controlling, and regulating the energy harvesting system with a thermoelectric cell. The model widely described in the literature, assuming a constant temperature level on both sides of the cell, has been modified to take into account the thermal resistance of heat exchangers that are inseparable parts of nearly every device of this kind. The results and conclusions from the solutions of equations forming a formalized record of the proposed method, the assumed approach to modeling, used physical phenomena and sensitivity analysis of the impact of the tested parameters on the system operation was presented. The calculations were made for the data of a selected thermoelectric generating cell available on the market.

An alternative energy flow model for analysis and optimization of heat transfer systems

The common existence of heat transfer phenomena in energy utilization systems highlights the importance of appropriate design and optimization methods of heat transfer systems to an unprecedented level. In this contribution, we defined an alternative thermal resistance for heat exchangers based on the inlet temperature difference of hot and cold fluids, which was influenced by such two aspects as the finite heat capacity rates of hot and cold fluids and the entransy dissipation-based thermal resistance of heat exchangers. Meanwhile, the corresponding energy flow models are proposed to analyze the heat transfer performance of both individual heat exchangers and three basic heat exchanger networks, i.e. series, parallel and multi-loop by the thermo-electrical analogy method, which offer the solution for constructing the energy flow models of any heat transfer system. For validation, we constructed the energy flow model of a double-loop thermal management system. On this basis, the heat transfer characteristic was described on the system level and the overall system constraints were obtained by the Kirchhoff’s Law without introducing any intermediate variables. With these system constraints, the optimization equations for the system were derived theoretically by applying the Lagrange multiplier method. Simultaneously solving these equations gave the optimal thermal conductances of each heat exchanger and the best allocations of heat capacity rate directly, which benefited energy conservation. That is, the energy flow model is reliable and convenient for the analysis and optimization of heat transfer systems.

Thermodynamic analysis of an idealised solar tower thermal power plant

In the real solar tower thermal power system, it is widely acknowledged that the thermodynamic irreversibility, such as convective and radiative loss on tower receiver, and thermal resistance in heat exchangers, is unavoidable. With above factors in mind, this paper presents an ideal model of the solar tower thermal power system to analyze the influence of various parameters on thermal and exergy conversion efficiencies, including receiver working temperature, concentration ratio, endoreversible heat engine efficiency and so forth. And therefore the variation of maximum thermal conversion efficiency in terms of concentration ratio and endoreversible heat engine efficiency could be theoretically obtained. The results indicate that raising the receiver working temperature could initially increase both thermal and exergy conversion efficiencies until an optimum temperature is reached. The optimum temperature would also increase with the concentration ratio. Additionally, the concentration ratio has a positive effect on the thermal conversion efficiency: increasing the concentration ratio could raise the conversion efficiency until the concentration ratio is extremely high, after which there will be a slow drop. Lastly, the endoreversible engine efficiency also has significant influence on the thermal conversion efficiency, it will increase the thermal conversion efficiency until it reaches the maximum and optimum value, and then the conversion efficiency will drop dramatically.

Net Power Optimization of an Irreversible Otto Cycle Using ECOP and Ecological Function

This paper presents an analysis of an irreversible Otto cycle aiming to optimize the net power through ECOP and ecological function. The studied cycle operates between two thermal reservoirs of infinite thermal capacity, with internal irreversibilities derived from non-isentropic behavior of compression and expansion processes, irreversibilities from thermal resistance in heat exchangers and heat leakage from the high temperature reservoir to the low temperature reservoir. Analytical expressions are applied for the power outputs optimized by the ECOP, by the ecological function and by the maximum power criteria, in conjunction with a graphic analysis, in which some cycle operation parameters are analyzed for an increased comprehension of the effects of the irreversibilities in the optimized power.

Application of entransy in the analysis of HVAC systems in buildings

The main task of HVAC systems in the cooling condition is to remove heat from indoor environment to outdoor environment. HVAC systems are complex networks of various processes, e.g., heat transfer, heat–work conversion, heat–humidity conversion, etc. These processes correspond to equipment such as heat exchangers, indoor cooling terminals, heat pumps, and cooling towers. Single analysis method or thermal parameter could hardly describe all the processes in an HVAC system. Exergy destruction refers to the loss of heat–work conversion ability. Reducing exergy destruction indicates less supplied exergy (input work) of HVAC systems. Entransy is a new parameter defined as heat transfer ability. Entransy dissipation refers to the loss of heat transfer ability. When the purpose of heat transfer is cooling or heating, entransy analysis is a direct method for optimizing heat transfer processes. Loss of HVAC system is mainly in heat transfer process. The entransy dissipation extremum principle or the minimum thermal resistance principle is suitable for analyzing heat transfer process in HVAC system. For indoor cooling, reducing entransy dissipation will increase chilled water temperature. Flow unmatched coefficient ξ represents an increase of thermal resistance of heat exchanger if the calorific capacities of the fluids are different.

New Configurations of Micro Plate-Fin Heat Sink to Reduce Coolant Pumping Power

The thermal resistance of heat exchangers has a strong influence on the electric power produced by a thermoelectric generator (TEG). In this work, a real TEG device is applied to three configurations of micro plate-fin heat sink. The distance between certain microchannels is varied to find the optimum heat sink configuration. The particular focus of this study is to reduce the coolant mass flow rate by considering the thermal resistances of the heat sinks and, thereby, to reduce the coolant pumping power in the system. The three-dimensional governing equations for the fluid flow and the heat transfer are solved using the finite-volume method for a wide range of pressure drop laminar flows along the heat sink. The temperature and the mass flow rate distribution in the heat sink are discussed. The results, which are in good agreement with previous computational studies, show that using suggested heat sink configurations reduces the coolant pumping power in the system.

Ground source heat pumps: observations from United Kingdom ground thermal response tests

The United Kingdom is experiencing a period of rapid growth in the use of ground source heat pump systems. Most installations in the United Kingdom use vertical ‘borehole’ heat-exchanger arrays, the design of which depends on four parameters: formation thermal conductivity, formation heat capacity, heat-exchanger resistance and heat-exchanger grout material heat capacity. Conventionally, two of these parameters (conductivity and resistance) are obtained from a thermal response test carried out on a trial heat exchanger at the site of interest by fitting thermal response data to classical line-source heat conduction theory. This test method gives no information on the heat capacities of the formation and grout material and requires an assumption about the former to enable the heat-exchanger resistance parameter to be extracted. In this work, a new method is developed for extracting all four parameters using a trust-region search algorithm in conjunction with a detailed numerical model of the test heat exchanger. Results give excellent agreement between the fitted-model predictions of heat-exchanger outlet water temperature and measured outlet water temperature for 13 test cases. A further advantage of the method developed here is that it can be used with data sets that contain disturbances and discontinuities. Practical applications: Most of the ground source heat pump installations in the United Kingdom use vertical ‘borehole’ heat-exchanger arrays. The design of these arrays requires information about the rock formation thermal conductivity and volume specific heat capacity and the borehole heat-exchanger thermal resistance and grout material volume specific heat capacity. These design parameters are usually obtained from a thermal response test carried out on a trial heat exchanger at the site of interest. In this work, thermal response test results from 13 UK sites are presented and a new method for obtaining the four design parameters is developed and proposed.

Experimental prediction of total thermal resistance of a closed loop EAHE for greenhouse cooling system

The design of an earth to air heat exchanger (EAHE) requires knowledge of its total thermal resistance ( R Tot ) for heating and cooling applications. In this research, a 47 m long horizontal, 56 cm nominal diameter U-bend buried galvanized was studied experimental EAHE used for the determination and evaluation of thermal properties of heat exchanger. This system was designed and installed in the Solar Energy Institute, Ege University, Izmir, Turkey. Based on the experimental results, generalized relationships were developed for predicting of thermal resistance of the heat exchanger. Average total heat exchanger thermal resistance was estimated to be 0.021 K-m/W as a constant value under steady state condition.

Analysis of entransy dissipation in heat exchangers

The optimization of heat exchangers is an important topic. Entropy generation is used to describe the irreversibility of heat transfer processes and the principle of minimum entropy generation is sometimes used to optimize heat exchanger designs. This paper defines the heat exchanger thermal resistance based on its entransy dissipation and analyses various heat exchangers. Entropy generation analyses are also presented for comparison. The results indicate that the minimum entransy-dissipation-based thermal resistance always corresponds to the highest heat transfer rate, while the design with the minimum entropy generation is not always related to the design with the highest heat transfer rate.

Supervision of the thermal performance of heat exchanger trains

In oil refining, heat exchanger networks are employed to recover heat and therefore save energy of the plant. However, many heat exchangers in crude oil pre-heat trains are under high risk of fouling. Under fouling conditions, the thermal performance of heat exchangers is continuously reduced and its supervision becomes an important task. The large number of heat exchangers in pre-heat trains and the change of operation conditions and feedstock charges make the daily supervision a difficult task. This work applies an approach to follow the performance of heat exchangers [M.A.S. Jerónimo, L.F. Melo, A.S. Braga, P.J.B.F. Ferreira, C. Martins, Monitoring the thermal efficiency of fouled heat exchangers – A simplified method, Experimental Thermal and Fluid Science 14 (1997) 455–463] and extends it to monitor the whole train. The approach is based on the comparison of measured and predicted heat exchanger effectiveness. The measured value is computed from the four inlet and outlet temperatures of a heat exchanger unit. The predicted clean and dirty values of effectiveness are calculated from classical literature relations as a function of NTU and of heat capacity ratio ( R). NTU and R are continuously adjusted according to mass flow rate changes. An index of fouling is defined for the whole network and the results show the performance degradation of the network with time. The work also suggests that Jerónimo’s index of fouling can be used to estimate the fouling thermal resistance of heat exchangers.

Enhanced Heat Transfer Using Porous Carbon Foam in Cross Flow—Part I: Forced Convection

Cogeneration of heat and power has become standard practice for many industrial processes. Research to reduce the thermal resistance in heat exchangers at the gas/solid interface can lead to greater energy efficiency and resource conservation. The main objective of this experimental study is to quantify and compare the heat transfer enhancement of carbon foam and aluminum fins. The study measures the heat transfer rate and pressure drop from a heated vertical pipe, with and without porous medium, in forced convection. The largest increase in Nusselt number was achieved by aluminum fins, which was about three times greater than the best carbon foam case.

A new approach to optimum design in thermoelectric cooling systems

A thermoelectric cooling system usually consists of a thermoelectric cooler (TEC) and heat exchangers at the cold side and the hot side. Heat exchangers in TEC systems are designed to minimize their thermal resistance under restrictions such as the size of the system and heat transfer method, because as the thermal resistances of heat exchangers increase, the performance of TEC systems decreases. In the TEC system in which thermal resistance of heat exchangers is minimized, the optimum TEC is analyzed to maximize the coefficient of performance (COP) and the exergetic efficiency in absorbing a given heat load. The effects of the thermal resistance of heat exchangers on the performance of the TEC system and the design parameters of that system are also investigated. To discuss these matters, dimensionless entropy flow equations of the TEC system are introduced, and the COP and the exergetic efficiency are expressed as a function of dimensionless quantities by using these dimensionless equations.