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The current study aims to perform a stochastic analysis on microturbine compact recuperators to evaluate the impact of uncertainties in design parameters on their cost and volume, by using two different probabilistic approaches: Monte Carlo (MC) and Response Sensitivity Analysis (RSA). These two methods have been developed in Matlab® and then coupled with CHEOPE (Compact Heat Exchanger Optimisation and Performance Evaluation) software, which allows to analyze two different types of recuperators, used in microturbine applications: the furnace-brazed plate-fin type and the welded primary surface type. This paper focuses on an analysis of plate-fin type recuperators, for which the cost function adopted was tuned and verified in a previous study. Three main parameters of the recuperator have been considered as uncertain: effectiveness, hot side and cold side pressure drops. The uncertainties associated with these three parameters are based on industrial knowledge. The aforementioned stochastic methods have been used to propagate such uncertainties on the relevant outputs, such as cost and volume, allowing us to evaluate the least expensive and the most compact recuperator among those analysed.

This paper reports an experimental study on the heat transfer characteristics of a nanofluid consisting of ZnO/water/ethylene glycol (EG) and TiO2/water/ /ethylene glycol. In this study, the base fluids of ethylene glycol (EG):water (W) with volume fractions of 30:70, 40:60, and 50:50 were prepared, and 0.2 to 1.0 volume fractions of ZnO and TiO2 nanofluids were used as a cold side fluid. The prime objective of this study is to identify the effects of nanofluid concentration and three different hot fluid inlet temperatures viz., 55, 65 and 75?C C on the heat transfer enhancement of cold side fluid. The results are compared with base fluids and the percentage increase of the Nusselt number because of nanoparticle addition is noted both experimentally and theoretically. The results showed that at the hot fluid inlet temperature of 75?C, the increase in the Nusselt number is maximum with volume concentrations of 0.6 and 0.8% for ZnO and TiO2 nanofluids, respectively. The corresponding maximum Nusselt number enhancements are about 11.5 and 21.4%, respectively, for the base fluid volume fraction of 30:70 (EG:W). There is good agreement between the results calculated from experimental values and the correlation.

In this paper a two-dimensional numerical simulation of a steady incompressible and turbulent model has been carried out to study the effects of vortex generators in a compact heat exchanger in a curvilinear coordinate system. The mesh which is applied in this study is boundary fitted and has been smoothed by a Laplace operator. Experimental data of a former study has been applied to validate the numerical results. The effects of geometrical variation are studied by adjusting vortex generators’ inclination and relative cross location. The major issue of this study is the optimal trade-off by selecting an optimal geometric, considering the opposite influences of geometrical variation on Nusselt number and pressure drop. In this paper a two-dimensional numerical simulation of a steady incompressible and turbulent model has been carried out to study the effects of vortex generators in a compact heat exchanger in a curvilinear coordinate system. The mesh which is applied in this study is boundary fitted and has been smoothed by a Laplace operator. Experimental data of a former study has been applied to validate the numerical results. The effects of geometrical variation are studied by adjusting vortex generators’ inclination and relative cross location. The major issue of this study is the optimal trade-off by selecting an optimal geometric, considering the opposite influences of geometrical variation on Nusselt number and pressure drop.

U radu je provedena termodinamička i hidrodinamička analiza kompaktnih izmjenjivača topline različitih konstrukcijskih tipova s ciljem optimizacije njihove geometrije. Termodinamička analiza je ostvarena numeričkim putem uz provjeru matematičkog modela i pripadajućih mu numeričkih simulacija eksperimentalnim putem. U zračnom tunelu otvorenog tipa, namjenski razvijenom za potrebe ovog istraživanja, mjereni su temperatura i maseni protoci radnih medija na cijevnom lamelnom i na dva lamelna izmjenjivača topline s mikrokanalima. Usporedba dobivenih rezultata mjerenja za navedene izmjenjivače topline je ostvarena u odnosu na iskoristivost topline i prosječni toplinski tok po ukupnoj masi i volumenu izmjenjivača. Za numeričku analizu su prema izmjenjivačima topline korištenim u eksperimentu razvijeni modeli odgovarajuće geometrije u smjeru strujanja vode i strujanja zraka. Primjenom složenijeg matematičkog modela zrak/voda, koji uključuje provođenje topline kroz stijenku cijevi i pad temperature vode u aksijalnom smjeru, postignuti su točniji rezultati u odnosu na modele s pretpostavljenom konstantnom temperaturom stijenke cijevi. Numerički trodimenzijski modeli izmjenjivača topline su riješeni primjenom metode kontrolnih volumena. Optimizacija geometrije kompaktnih izmjenjivača topline ostvarena je za lamelni izmjenjivač topline s mikrokanalima koji pripada vrhunskim tehnologijama u tehnici grijanja, klimatizacije i ventilacije. Primijenjena je metoda odzivnih ploha za model izmjenjivača s četiri izabrana parametra od kojih su tri geometrijska, a jedan odražava izabrani pogonski uvjet. Na osnovi dobivenih rezultata optimizacije formirane su smjernice za izbor optimalne geometrije lamelnih izmjenjivača topline s mikrokanalima sa stajališta termodinamičkih i hidrodinamičkih karakteristika., In this paper thermodynamical and hydraulical analysis of compact heat exchangers of different construction types has been performed in order to optimize their geometry. Thermodynamical analysis has been carried out numerically along with the evaluation of the mathematical model and corresponding numerical simulations through the experiment. In an open circuit wind tunnel developed on purpose for this investigation, the measurement of working media temperatures and mass flows for fin-and-tube and two heat exchangers with microchannel coil has been accomplished. The comparison of the obtained measurement results for the mentioned heat exchangers has been performed in relation to the heat transfer effectiveness and specific heat transfer rate per total mass and heat exchanger’s volume. For the purpose of numerical analysis and in accordance with the heat exchangers used for experiments, models with adequate geometry in direction of both air and water flow have been developed. With utilization of air/water side model, more accurate results have been achieved in relation to the model that assumes constant temperature or constant heat flux on the pipe wall. Numerical 3D models of heat exchangers have been solved using the finite volume method. The geometry optimization of compact heat exchangers has been accomplished for the heat exchanger with microchannel coil that belongs to state of the art technologies in heating, ventilating and air conditioning industry. The response surface method has been used for the heat exchanger model with four chosen parameters three of them being geometrical and one reflecting the chosen operating condition. From the thermodynamical and hydraulical point of view and based on the obtained optimization results, directions have been given for the choice of optimal geometry of the heat exchanger with microchannel coils.

In this paper, an analysis of heat transfer and fluid flow in a wavy fin-and-tube heat exchanger has been performed followed with a comparative study of different algorithms used for the heat exchanger optimization. Three- dimensional (3D) numerical simulation results of a circular tube heat exchanger were used in multivariate optimization that included geometrical parameters that, as predicted, significantly influence thermodynamic and hydrodynamic behavior of the heat exchanger system. A nonlinear regression model (polynomial function) was derived for two developed objective functions. Several algorithms for the optimization of heat exchanger have been used: two evolutionary algorithms (Genetic Algorithm and Simulated Annealing) and Pattern Search algorithm. The influences of inlet air velocity, as well as fin pitch and wavy angle on heat transfer and pressure drop conditions have been studied. The comparison of results has been made along with the analysis and conclusions.

Heat exchangers play significant role in refrigeration and air conditioning systems. Ongoing research aims to improve, or at least maintain, the system performance while reducing the size, weight and cost of the heat exchangers. This in turn leads to lower system refrigerant charge and reduced environmental impact. Using heat exchangers with small tube diameters (less than 5 mm) instead of large tube diameters has been shown to be a promising solution to meet the aforementioned goals. However, shifting towards mall tube diameters requires in-depth analysis and optimization of several heat exchanger design parameters. This paper presents a multi-objective optimization of a residential air source heat pump system using genetic algorithms with a particular focus on the use of small diameter tubes in the heat exchangers. The objectives are to minimize the heat exchangers’ cost and maximize the system performance. The goal of this study is to determine the potential material savings and cost reduction when using tube diameters between 3 mm and 5 mm in the heat exchangers. In addition to the tube diameters, multiple fin types, tube spacing and fin densities are also investigated. The optimization is carried out for R-410A, and a lower-GWP alternative, R-32. The system utilizing the improved heat exchanger designs has a cost reduction of 50% in comparison to the baseline system. Also, the improvements in the system’s COP and the system charge reduction are around 20% and 35%, respectively.Â

In this paper, numerical, curvilinear and turbulent model has been used to investigate the effect of vortex generator's longitudinal displacement on heat transfer and fluid flow in different Reynolds numbers ranging from 500 to 3000. The numerical model has been validated with experimental results of a former study, which is resembled in a particular case. Numerical simulation shows that the vorticity enhancement would increase both Nusselt number and pressure drop. Proposed trend is not constant and the pattern in which parameters change is dependent on Reynolds number. Finally, a conjugated optimization of pressure drop and the Nusselt number has been suggested based on the order of parameter changes.

In this paper numerical and experimental analysis of compact heat exchanger has been done. A fin and tube heat exchanger composed of four rows of circular tubes has been installed in an open circuit wind tunnel. Air and water were used as working fluids. Same geometry and parameter setup has been analyzed using commercial fluid flow and a heat transfer solver Fluent. Air/water side model has been used because it gives more accurate results than simpler model that includes only air side flow. Comparison between numerical model and experimental results has been accomplished with respect of air and water temperature. It has been shown that numerical model results coincide well with the experimental data, with deviations within an acceptable range. Therefore, used air/water side model can be used for further investigation of more efficient compact heat exchanger geometry.

Fouling problems cannot be avoided in many heat exchanger operations, and it is necessary to introduce defensive measures to minimize fouling and the cost of cleaning. The fouling control measures used during either design or operation must be subjected to a thorough economic analysis, taking into consideration all the costs of the fouling control measures and their projected benefits in reducing costs due to fouling. Under some conditions, nearly asymptotic fouling resistances can be obtained, and this suggests a somewhat different approach to the economics. Fouling is a generic term for the deposition of foreign matter on a heat transfer surface. Deposits accumulating in the small channels of a compact heat exchanger affect both heat transfer and fluid flow. Fouling deposits constricting passages in a compact heat exchanger are likely to increase the pressure drop and therefore reduce the flow rate. Reduced flow rate may be a process constraint; it reduces efficiency and increases the associated energy use and running costs. Maintenance costs will also increase. Fouling remains the area of greatest concern for those considering the installation of compact heat exchangers. The widespread installation of compact heat exchangers has been hindered by the perception that the small passages are more strongly affected by the formation of deposits. In this paper different types of fouling and treatment are presented.

U radu je provedena termodinamička i hidrodinamička analiza kompaktnih izmjenjivača topline različitih konstrukcijskih tipova s ciljem optimizacije njihove geometrije. Termodinamička analiza je ostvarena numeričkim putem uz provjeru matematičkog modela i pripadajućih mu numeričkih simulacija eksperimentalnim putem. U zračnom tunelu otvorenog tipa, namjenski razvijenom za potrebe ovog istraživanja, mjereni su temperatura i maseni protoci radnih medija na cijevnom lamelnom i na dva lamelna izmjenjivača topline s mikrokanalima. Usporedba dobivenih rezultata mjerenja za navedene izmjenjivače topline je ostvarena u odnosu na iskoristivost topline i prosječni toplinski tok po ukupnoj masi i volumenu izmjenjivača. Za numeričku analizu su prema izmjenjivačima topline korištenim u eksperimentu razvijeni modeli odgovarajuće geometrije u smjeru strujanja vode i strujanja zraka. Primjenom složenijeg matematičkog modela zrak/voda, koji uključuje provođenje topline kroz stijenku cijevi i pad temperature vode u aksijalnom smjeru, postignuti su točniji rezultati u odnosu na modele s pretpostavljenom konstantnom temperaturom stijenke cijevi. Numerički trodimenzijski modeli izmjenjivača topline su riješeni primjenom metode kontrolnih volumena. Optimizacija geometrije kompaktnih izmjenjivača topline ostvarena je za lamelni izmjenjivač topline s mikrokanalima koji pripada vrhunskim tehnologijama u tehnici grijanja, klimatizacije i ventilacije. Primijenjena je metoda odzivnih ploha za model izmjenjivača s četiri izabrana parametra od kojih su tri geometrijska, a jedan odražava izabrani pogonski uvjet. Na osnovi dobivenih rezultata optimizacije formirane su smjernice za izbor optimalne geometrije lamelnih izmjenjivača topline s mikrokanalima sa stajališta termodinamičkih i hidrodinamičkih karakteristika. In this paper thermodynamical and hydraulical analysis of compact heat exchangers of different construction types has been performed in order to optimize their geometry. Thermodynamical analysis has been carried out numerically along with the evaluation of the mathematical model and corresponding numerical simulations through the experiment. In an open circuit wind tunnel developed on purpose for this investigation, the measurement of working media temperatures and mass flows for fin-and-tube and two heat exchangers with microchannel coil has been accomplished. The comparison of the obtained measurement results for the mentioned heat exchangers has been performed in relation to the heat transfer effectiveness and specific heat transfer rate per total mass and heat exchanger’s volume. For the purpose of numerical analysis and in accordance with the heat exchangers used for experiments, models with adequate geometry in direction of both air and water flow have been developed. With utilization of air/water side model, more accurate results have been achieved in relation to the model that assumes constant temperature or constant heat flux on the pipe wall. Numerical 3D models of heat exchangers have been solved using the finite volume method. The geometry optimization of compact heat exchangers has been accomplished for the heat exchanger with microchannel coil that belongs to state of the art technologies in heating, ventilating and air conditioning industry. The response surface method has been used for the heat exchanger model with four chosen parameters three of them being geometrical and one reflecting the chosen operating condition. From the thermodynamical and hydraulical point of view and based on the obtained optimization results, directions have been given for the choice of optimal geometry of the heat exchanger with microchannel coils.

The first Molten Salt Reactor (MSR) was designed and tested at Oak Ridge National Laboratory (ORNL) in the 1960's, but recent technological advancements now allow for new components, such as heat exchangers, to be created for the next generation of MSR's and molten salt-cooled reactors. The primary (fuel salt-to-secondary salt) heat exchanger (PHX) design is shown here to make dramatic improvements over traditional shell-and-tube heat exchangers when changed to a compact heat exchanger design. While this paper focuses on the application of compact heat exchangers on a Molten Salt Reactor, many of the analyses and results are similarly applicable to other fluid-to-fluid heat xchangers. The heat exchanger design in this study seeks to find a middle-ground between shell- and-tube designs and new ultra-efficient, ultra-compact designs. Complex channel geometries and microscale dimensions in modern compact heat exchangers do not allow routine maintenance to be performed by standard procedures, so extended surfaces will be omitted and hydraulic diameters will be kept in the minichannel regime (minimum channel dimension between 200 μm and 3 mm) to allow for high-frequency eddy current inspection methods to be developed. High aspect ratio rectangular channel cross-sections are used. Various plant layouts of smaller heat exchanger banks in a "modular" design are introduced. FLUENT was used within ANSYS Workbench to find optimized heat transfer and hydrodynamic performance. With similar boundary conditions to ORNL's Molten Salt Breeder Reactor's shell-and-tube design, the compact heat exchanger interest in this thesis will lessen volume requirements, lower fuel salt volume, and decrease material usage. Master of Science

In this paper an analysis of heat transfer and fluid flow of compact heat exchanger has been done. 3D numerical simulation results of a heat exchanger with microchannel coil were compared with experimental results carried out in this research. Microchannel coil composed of flat tubes with rectangular microchannels of different geometries has been installed in an open circuit wind tunnel. Air and distilled water were used as working fluids in heat exchanger with single phase heat transfer. Same geometry and parameter setup have been analyzed with commercial fluid flow and a heat transfer solver Fluent. Air/water model has been used. Good agreement between experimentally measured and calculated results has been accomplished. The influence of different microchannel geometry on heat transfer effectiveness and pressure drop have been studied numerically. Comparison of results has been made accompanied by the commentaries and final conclusions.

Compact heat exchangers are usually characterized by a large heat transfer surface per unit of volume. These characteristics are useful when thermal energy between two or more fluids must be exchanged without mixing. Most compact heat exchangers are liquid-to-air heat exchangers, with approximately 85% of the total thermal resistance occurring on the air side of the heat exchanger. To reduce the space and weight of a compact heat exchanger, augmentation strategies must be proposed to reduce the air side resistance. However, before any strategies to augment the air side heat transfer can be proposed, a thorough insight of the current mechanisms that govern air side heat transfer is required. The tube wall heat transfer results presented in this paper were obtained both experimentally and computationally for a typical compact heat exchanger design. Both isothermal and constant heat flux tube walls were studied. For the experimental investigation, a scaled-up model of the louvered fin-tube wall was tested in a flow facility. Although computational results for the isothermal tube wall are shown, control of the experimental isothermal tube wall proved to be unrealistic and only heat transfer measurements along the constant heat flux tube wall were made. For the constant heat flux tube wall, reasonable agreement has been achieved between the measurements and the steady, three-dimensional computational predictions. The results of the study showed that high heat transfer coefficients existed at the entrance to the louver array as well as in the louver reversal region. Vortices created at the leading edge of the louvers augmented heat transfer by thinning the tube wall boundary layer. Results indicate that an augmentation ratio of up to 3 times can occur for a tube wall of a louvered fin compact heat exchanger as compared to a flat plate. Master of Science