Design sensitivity analysis of a plate-finned air-cooled condenser for waste heat recovery ORCs

The study is related to the design sensitivity analysis of a plate-finned tube bundle V-shaped air-cooled condenser design problem for a range of representative low-temperature waste heat recovery Organic Rankine Cycle (ORC) cases. An iterative design model is implemented which reveals the thermodynamic and geometric design error margins that occur when different in-tube prediction methods are used. 19 condensation heat transfer correlations are used simultaneously within arrays of geometric and thermodynamic variables. Through attained 19 different convective coefficients, a design sensitivity on the calculated overall heat transfer coefficient, total transferred heat, degree of subcooling, required tube and fin material amount, air- and refrigerant-side pressure drops is reported

Design sensitivity analysis of using various in-tube condensation correlations for an air-cooled condenser for ORCs

The study is related to the evaluation of using 19 condensation heat transfer correlations in an annular finned horizontal round tube V-shaped air-cooled condenser design problem for a representative low-temperature waste heat recovery Organic Rankine Cycle (ORC) case. The condensation is realized through cold air provided by the fan suction at a mass flow rate of 90,35 kg/s, whereas the working fluid mass flow rate is 7,8 kg/s. The considered condensation temperature is 40°C which corresponds to a saturation pressure of 1,17 bar. The ambient air is considered to be 15°C. The investigated working fluid is SES36. For a given set of geometrical constraints, an iterative condenser design model is implemented. All considered correlations are applied separately for the same boundary conditions. The design sensitivity on the overall heat transfer coefficient, total transferred heat, required fan power, air- and refrigerant-side pressure drops is assessed. By those means, the engineering error margin of using different calculation tools in designing air-cooled condensers for ORC is reported

Experimental investigation at sub- and supercritical conditions of a helical coil heat exchanger particularly designed for a small-scale solar ORC

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Ultra high heat flux cooling provided by flow boiling in microscale with enhancements using nanostructured surfaces

Due to their heat transfer efficiency and compact implementation methods, the use of flow boiling via plain and modified microchannels for cooling solutions gained a significant importance in the last decade. The increasing need for more efficient cooling solutions in various fields of micro scale cooling such as aerospace, microreactors, automotive industry, micropropulsion, fuel cells, drug delivery systems, biological and chemical applications is motivating researchers to investigate the physics behind the micro scale flow boiling phenomena. The proposed study aims to make a contribution to the literature in the related field by filling the gap of scientific knowledge about the microchannel flow boiling heat transfer capabilities at ultra high mass fluxes, under unstable boiling conditions and with microchannels having inner wall surface enhancements via nanostructure coating. The present thesis study and results of related experiments are divided into three main parts: ultra high mass flux flow boiling experiments, the effect of inlet restrictions and tube size on premature critical heat flux in microchannels and flow boiling heat transfer enhancement via coating polyhydroxyethylmethacrylate (pHEMA) on inner microtube walls. In the first part, microchannels having ~250 μm and ~500 μm hydraulic diameters were tested at various ultra high mass fluxes values and different heated length for forcing the conventional heat removal limits of flow boiling via microchannels. De-ionized water was used as working fluid and test section was heated with Joule heating. Wall temperatures for each case were recorded and exit qualities were calculated. The resulting CHF boiling curve demonstrates cooling rates (>30000W/cm2) that were never achieved by flow boiling in microchannels could be obtained. In the second part of the present study, useful information about premature CHF phenomena was provided. The study offers a parametric comparative investigation. Experimental data are obtained from microtubes having 250~μm and 685~ μm inner diameters, which were tested at low mass fluxes (78.9-276.3 kg/m2s) to reveal potential boiling instability mechanisms. Moreover, inlet restrictions were introduced to the system for observing their effect in mitigating unstable boiling conditions and extending the boiling curve. De-ionized water was used as a coolant, while microtubes having 5,65 cm heated length were heated by Joule heating. Furthermore, Fast Fourier Transform (FFT) of the deduced data is performed for revealing the frequency correlations of the every obtained temperature and pressure oscillations before and just before the premature dryout condition. The results show the inlet restrictions have a significant effect on reducing the unstable boiling fluctuations and the proposed FFT method was proved to be a useful tool to detect premature dryout before it occurs. In the third part, flow boiling heat transfer experiments were conducted on microtubes (inner diameter of ~ 250 μm, ~500 μm and ~1 mm) with a constant heated length 2 cm and with enhanced inner surfaces having deposited polyhydroxyethylmethacrylate (pHEMA), which extends the boiling curve, increases the heat transfer surface area, and provides additional nucleation sites. De-ionized water was utilized as the working fluid and test section was heated by Joule heating in this study. Nanostructures on the microtube walls were coated through initiated chemical vapor deposition (iCVD) technique. A significant extension in CHF boiling curve and increase in heat transfer were observed with nanostructure-enhanced surfaces compared to the plain surface counterparts for two relatively high mass velocities, namely, 10000 kg/m2s and 13000 kg/m2s

Experimental investigation of a forced convection heat transfer of the organic fluid R-125 at supercritical pressures and under organic rankine cycle conditions

The organic Rankine cycle (ORC) is a suitable technology for utilizing low-grade temperature heat sources of ~100 °C from various industry processes. In the ORC cycle an organic fluid with a lower boiling point is used as a working medium. The performance of the ORCs has advanced significantly in the last decades. However, there is still a possibility of improving the efficiency of this cycle. The supercritical heat transfer in the heat exchanger ensures better thermal match between the heating and working fluids temperatures glides in the heat exchanger. Hence, better understanding of the heat transfer phenomena to a fluid at supercritical state in a horizontal flow and in a large diameter tube is of great importance. Therefore, the tests are performed in a counter-current tube-in-tube test section positioned horizontally with a total length of 4 m and a tube diameter of 0.0286 m. R-125 is used as a working fluid in the experiments. During the measurements the temperature of the heating fluid was 90 °C, the mass flow rate and the pressure of the working fluid R-125 was in the range of 0.2–0.3 kg/s and 38–55 bar respectively. Furthermore, results from the pressure and temperature measurements obtained at the inlet and at the outlet of the test section are reported. The results show that the overall heat transfer coefficient is influenced by the mass flow rate of the organic fluid. At pressures close to the critical pressure of R-125 higher values of the overall heat transfer coefficients are determined. Deteriorated heat transfer is not likely to occur at these operating conditions because the critical heat flux is higher than the one obtained from the measurements. A comparison between the experimental Nusselt number with heat transfer (Nusselt) correlations from the literature is done and the measurement points fall within the uncertainty ranges of both heat transfer correlations.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers

Design sensitivity analysis of using various flow boiling correlations for a direct evaporator in high-temperature waste heat recovery ORCs

High-temperature waste heat (250°C-400°C) sources being created by industrial operations such as metallurgical industry, incinerators, combustion engines, annealing furnaces, drying, baking, cement production etc. are being utilized in Organic Rankine cycle (ORC) waste heat recovery systems. Alongside indirect ORC evaporators having intermediate heat carrier loops, ORC waste heat recovery can also be done through a direct evaporator (e.g. tube bundles) applied on a heat source. In an evaporator design problem, the accuracy of the design method has a significant impact on the end result. In that manner, for revealing the design accuracy error margin of using various flow boiling heat transfer methods, a design sensitivity analysis is performed by means of using 13 different flow boiling heat transfer correlations. All correlations are implemented separately into an iterative evaporator calculation and the resulting sizing solutions are compared for a representative high-temperature waste heat recovery evaporator case. The volumetric flow rate of the waste heat is 80000 Nm³/h and the inlet temperature is 375°C. The considered working fluid is cyclopentane and the deduced optimal evaporation temperature (OET) is 227°C. The minimum corresponding total transferred heat in the evaporator is at least 3,5 MW in all calculations

Design and rating of an evaporator for waste heat recovery organic rankine cycles using SES36

The paper presents a design and rating study of a 4MW evaporator having plain horizontal carbon steel tubes having diameters of 25,4 mm, 31,8 mm and 38 mm, to be used in waste heat recovery via Organic Rankine cycle (ORC). SES36 is chosen as working fluid due to its low boiling point, which makes it suitable for low-grade waste heat recovery with subcritical ORCs. Waste heat carrier industrial air arrives at the evaporator bundle at 280°C. Inlet temperature of the working fluid is 40°C and the evaporation occurs at 125°C and 1,09 MPa. Furthermore, a design sensitivity analysis is made by means of using 13 different in-tube flow boiling correlations. The resulting design and rating parameters yielded by each correlation are compared to each other. By those means, a design error margin of various thermo-hydraulic heat exchanger parameters is revealed, when different in-tube flow boiling heat transfer calculation methods are used. The change in the error margins are investigated with respect to changing tube outer diameter, tube wall thickness, fin density and tube layout (staggered and inline)

Experimental assessment of a helical coil heat exchanger operating at subcritical and supercritical conditions in a small-scale solar organic rankine cycle

In this study, the performance of a helical coil heat exchanger operating at subcritical and supercritical conditions is analysed. The counter-current heat exchanger was specially designed to operate at a maximal pressure and temperature of 42 bar and 200 °C, respectively. The small-scale solar organic Rankine cycle (ORC) installation has a net power output of 3 kWe. The first tests were done in a laboratory where an electrical heater was used instead of the concentrated photovoltaic/thermal (CPV/T) collectors. The inlet heating fluid temperature of the water was 95 °C. The effects of different parameters on the heat transfer rate in the heat exchanger were investigated. Particularly, the performance analysis was elaborated considering the changes of the mass flow rate of the working fluid (R-404A) in the range of 0.20–0.33 kg/s and the inlet pressure varying from 18 bar up to 41 bar. Hence, the variation of the heat flux was in the range of 5–9 kW/m2. The results show that the working fluid’s mass flow rate has significant influence on the heat transfer rate rather than the operational pressure. Furthermore, from the comparison between the experimental results with the heat transfer correlations from the literature, the experimental results fall within the uncertainty range for the supercritical analysis but there is a deviation of the investigated subcritical correlations