An experimental analysis of flow boiling and pressure drop in a brazed plate heat exchanger for organic Rankine cycle power systems

Heat transfer and pressure drop during HFC refrigerant vaporisation inside a brazed plate heat exchanger

This paper presents the experimental heat transfer coefficients and pressure drop measured during HC-600a (isobutane), HC-290 (propane), and HC-1270 (propylene) vaporization inside a brazed plate heat exchanger (BPHE): the effects of heat flux, refrigerant mass flux, saturation temperature (pressure), evaporator outlet condition, and fluid properties are investigated. The experimental tests include 172 vaporization runs carried out at three different saturation temperatures (10, 15, and 20 °C) and four different evaporator outlet conditions (outlet vapor quality around 0.80 and 1.00, outlet vapor super-heating around 5 and 10 °C). The refrigerant mass flux ranges from 6.6 to 23.9 kg m−2 s−1 and the heat flux from 4.3 to 19.6 kW m−2. The heat transfer and pressure drop measurements have been complemented with IR thermography analysis in order to quantify the portion of the heat transfer surface affected by vapor super-heating. The heat transfer coefficients show great sensitivity to heat flux, evaporator outlet condition and fluid properties and weak sensitivity to saturation temperature (pressure). The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on refrigerant mass flux. HC-1270 exhibits heat transfer coefficients 6–12% higher than HC-290 and 35–50% higher than HC-600a and frictional pressure drops 5–10% lower than HC-290 and 60% lower than HC-600a. The experimental heat transfer coefficients are compared with two well-known correlations for nucleate boiling and a linear equation for frictional pressure drop is proposed.

This paper presents the experimental heat transfer coefficients and pressure drop measured during HC-600a (Isobutane), HC-290 (Propane) and HC-1270 (Propylene) vaporisation inside a small brazed plate heat exchanger: the effects of heat flux, refrigerant mass flux, saturation temperature (pressure), outlet conditions and fluid properties are investigated. The experimental tests include 172 vaporisation runs carried out at three different saturation temperatures: 10, 15 and 20°C. The refrigerant mass flux ranges from 6.6 to 23.9 kg/m2s and the heat flux from 4.3 to 19.6 kW/m2. The heat transfer coefficients show great sensitivity to heat flux, outlet conditions and fluid properties and weak sensitivity to saturation temperature (pressure). The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on refrigerant mass flux. HC-1270 shows heat transfer coefficients 6–12% higher than HC-290 and 35–50% higher than HC-600a and frictional pressure drops 5–10% lower than HC-290 and 2.5 time lower than HC-600a. The experimental heat transfer coefficients are compared with two well-known equations for nucleate boiling and a correlation for frictional pressure drop is proposed.

Organic Rankine cycle (ORC) power systems are widely used for low-temperature heat recovery. The radial turbine is the most important component in the ORC system and its performance is significantly dependent on the working fluid. Therefore, the effect of different working fluid on the performance of radial turbine in ORC system for the low-temperature heat source (120~180 °C) was investigated. First, the feasibilities of different working fluids for 150 kW radial turbine using R600a as designed working fluid were investigated. The thermal aerodynamic calculation of R600a at design condition was conducted with evaporation temperature 95 °C and condensation temperature 38 °C. The operation parameters of 5 non-designed working fluids (R600, R245fa, R245ca, R123 and R601a) were selected in the same temperature variation range as R600a. Then, the performance of a radial turbine for six working fluids at design condition was analyzed, and their three-dimensional flow field performances were also obtained by CFD numerical simulation. Finally, to broaden the operation range of the radial turbine and investigate the matching relationship between operation parameters and different working fluids, the off-design performances of radial turbine using six working fluids were analyzed and the effect of rotation speed, inlet pressure and inlet temperature on the performance of radial turbine was obtained. Results show that the performance of the radial turbine using the designed working fluid R600a is the best with efficiency of 82.7 % and output power of 150.5 kW. For the non-designed working fluids, the comprehensive performance of R600 was also judged to be good owing to the highest efficiency (85.1 %) and a relatively high power output (115.8 kW) among the fluids tested. For other working fluids, the output power of the radial turbine decreased obviously. There exists an optimum rotation speed for different working fluids to ensure that the radial turbine has the highest efficiency. The inlet pressure has a larger effect on the performance of the radial turbine than that of the inlet temperature. The results can provide basic data for the optimization design and operation of radial turbines in the ORC system.

Refrigerant R134a vaporisation heat transfer and pressure drop inside a small brazed plate heat exchanger

The organic Rankine cycle (ORC) system in plants, powered by dual steam–water heat sources, has significant power generation potential and practical research value. Herein, the conditions of 700 kPa saturated steam and 650 kPa, 90 °C water heat source are considered. Four configurations of steam–water dual heat source waste heat recovery ORC systems are proposed. The independent parameters affecting the net output power of the system are obtained by developing a mathematical model and optimizing it using the particle swarm optimization method. The results show that the location of the pinch‐point temperature difference in various ORC loops and the allowable working pressure of the heat exchanger are determinants of independent parameters. The net output powers of the conventional dual‐loop ORC (CD‐ORC), single‐loop ORC (S‐ORC), split‐flow dual‐loop ORC (SFD‐ORC), and split‐flow triple‐loop ORC SFT‐ORC systems under the optimal design parameters are 2415.73, 2168.6, 2599.62, and 2716.75 kW, respectively. In addition, S‐ORC has the highest exergy efficiency of 55.17%. SFD‐ORC and SFT‐ORC have ≈48% exergy efficiency, and CD‐ORC has the lowest exergy efficiency of 45.33%.

Thermal and economic analysis on vehicle energy supplying system based on waste heat recovery organic Rankine cycle

For Steam Rankine Cycle (SRC), Organic Rankine Cycle (ORC) and Steam-Organic Rankine Cycle (S-ORC) power systems, in this paper, mathematical models are developed to explore the feasibility that combines the fluid-low temperature (150–350°C) waste heat steam and low-boiling point organic working fluids for power generation. Using the numerical models, we calculate and compare thermal efficiency, exergy efficiency, operation pressure, generating capacity, etc. of three power systems, namely SRC, ORC and S-ORC under the same heat source conditions. The results show that under the condition of 150–210°C heat source, ORC has the highest thermal efficiency, exergy efficiency and power generation; while at 210–350°C, the performance of the S-ORC has a distinct advantage. Its thermal efficiency and exergy efficiency are higher than those of the SRC and ORC power systems.

Experimental study on flow boiling heat transfer and pressure drop of R245fa/R141b mixture in a horizontal microfin tube

Numerical optimization of an injection volumetric expander for use in waste heat recovery organic Rankine cycle

Characterizing the performance of a single-screw expander in a small-scale organic Rankine cycle for waste heat recovery

Vaporisation of the low GWP refrigerant HFO1234yf inside a brazed plate heat exchanger

In this paper, we present an assessment of methods for estimating and comparing the thermodynamic performance of working fluids for organic Rankine cycle power systems. The analysis focused on how the estimated net power outputs of zeotropic mixtures compared to pure fluids are affected by the method used for specifying the performance of the heat exchangers. Four different methods were included in the assessment, which assumed that the organic Rankine cycle systems were characterized by the same values of: (1) the minimum pinch point temperature difference of the heat exchangers; (2) the mean temperature difference of the heat exchangers; (3) the heat exchanger thermal capacity ( U ¯ A ); or (4) the heat exchanger surface area for all the considered working fluids. The second and third methods took into account the temperature difference throughout the heat transfer process, and provided the insight that the advantages of mixtures are more pronounced when large heat exchangers are economically feasible to use. The first method was incapable of this, and deemed to result in optimistic estimations of the benefits of using zeotropic mixtures, while the second and third method were deemed to result in conservative estimations. The fourth method provided the additional benefit of accounting for the degradation of heat transfer performance of zeotropic mixtures. In a net power output based performance ranking of 30 working fluids, the first method estimates that the increase in the net power output of zeotropic mixtures compared to their best pure fluid components is up to 13.6%. On the other hand, the third method estimates that the increase in net power output is only up to 2.56% for zeotropic mixtures compared to their best pure fluid components.

An organic Rankine cycle (ORC) system with R123 working fluid has been utilised for generating electricity from low-temperature geothermal resources. The degree of superheated vapour warrants attention to be studied further. This is because the degree of superheated vapour is the last point to absorb heat energy from geothermal heat sources and influence the amount of expansion power produced by the expander. Therefore, achieving high ORC system efficiency requires a parameter of superheated vapour degree. This paper presents an experimental study on a binary cycle, applying R123 as the working fluid, to investigate the effect of variation in superheated vapour degree on the ORC efficiency. Geothermal heat sources were simulated with conduction oil as an external heat source to provide input heat to the ORC system. The temperature high inlet (TH in) evaporator was designed to remain at 120 °C during the experiment, while mass flow rate was adjusted to make superheated vapour variations, namely set at 278, 280, 282, 284, and 286 K. Furthermore, the effect was observed on heat transfer inlet, pinch, heat transfer coefficient, expander work output, isentropic efficiency, expander shaft power, power generation, thermal efficiency, and ORC efficiency. The experimental results showed that the mass flow rate nearly remained unchanged at different degrees of superheated vapour. The ranges of heat transfer inlet, pinch temperature, and heat transfer coefficient were 25.34–27.89 kJ/kg, 9.35–4.08 °C, 200.62–232.54 W/m2·K, respectively. In conclusion, ORC system efficiency can be triggered by various parameters, including the temperature on the exit side of the evaporator. The superheated vapour of R123 working fluid to higher temperatures has caused a decrease in ORC system efficiency due to the decrease in heat transfer inlets, although theoretically, the work total increased. Further investigation has found that the magnitude of the mass flow rate affects the behaviour of the components of the ORC system.

• High temperature flow boiling of seven working fluids is experimentally analyzed. • Two heat transfer mechanisms are identified among different working fluids. • A superposition model is developed for the heat transfer coefficient prediction. • The heat transfer coefficient of propane is up to 152% higher than that of R236fa. • Isobutane has the lowest frictional pressure drop among the seven working fluids. Organic Rankine cycle technology has gained worldwide acceptance as an efficient way to utilize low-grade heat sources. Plate heat exchangers are the most common type of heat exchanger employed as evaporators in small-scale organic Rankine cycle units, in which a high saturation temperature is the prevailing working condition. However, there is a lack of research on high temperature flow boiling in plate heat exchangers. This paper presents an experimental analysis on flow boiling heat transfer and pressure drop characteristics in a plate heat exchanger, and the development of prediction methods for the heat transfer coefficient and frictional pressure drop. Seven working fluids, R134a, R236fa, R245fa, R1234ze(E), R1233zd(E), propane and isobutane, were tested at the reduced pressures of 0.45, 0.55 and 0.65, corresponding to saturation temperatures ranging from 55 °C to 141 °C, and various mass fluxes. Two heat transfer mechanisms, nucleate boiling and thin-film evaporation, were identified in the heat transfer processes of the different working fluids, due to the diversity in their thermo-physical properties. Moreover, the results indicate that propane and isobutane have higher heat transfer coefficients than the other working fluids, while R236fa has the lowest heat transfer coefficient. The frictional pressure drops show the same characteristics for all the working fluids, increasing with the increase of the vapor quality and mass flux and the decrease of the saturation temperature. A superposition model presented in the paper achieves a good prediction for the heat transfer data, with a 12.8% mean absolute percentage deviation. A correlation developed in a previous work by the authors enables a prediction with an 11.1% mean absolute percentage deviation for the pressure drop data. The prediction methods presented in the paper will facilitate the modelling and design of plate heat exchanger evaporators in organic Rankine cycle units.