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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.

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