THEORETICAL AND NUMERICAL STUDY OF ORGANIC FLUID HEAT TRANSFER CHARACTERISTIC AT SUPERCRITICAL PRESSURE

• Main Author: Harmen
• Format: Dissertations
• Language: Indonesia
• Subjects: Teknik (Rekayasa, enjinering dan kegiatan berkaitan)
• Online Access: https://digilib.itb.ac.id/gdl/view/57023
• Institution: Institut Teknologi Bandung

Thermodynamically, the use of supercritical organic Rankine cycle technology for power generation from low-calorie energy sources results has higher efficiency, low destruction exergy, simple construction, and lower investment and operating costs compared to the conventional Rankine steam cycles and subcritical organic Rankine cycles. Supercritical ORC studies continue to be developed covering aspects of thermodynamic optimization, determination of working fluids, turbines/expanders, heat transfer devices, and economic aspects. The heat transfer process that occurs in the heat exchanger is the process of transferring energy from energy source to organic fluid which is used as a working fluid to drive a turbine/expander to generate power. At supercritical pressure this heat transfer process is better than a subcritical pressure. In the design of the heat exchanger, it is necessary to know the heat transfer characteristics of the working fluid to obtain optimal conditions. For subcritical conditions this heat transfer characteristic is available and mature for application. Unfortunately, the process of heat transfer under supercritical conditions is very complex and complicated due to the sharp changes in the thermodynamic and physical properties of the fluid under these conditions. Therefore, in this study is investigated the force convective heat transfer characteristics of supercritical fluids such as propane at pressures above the critical pressure or at the supercritical pressure. The availability of appropriate heat transfer characteristics for a particular organic fluid under specific conditions, an efficient heat exchanger can be produced to increase the performance and economic value of the supercritical ORC generating system. In this study, the formulation of a new Nusselt number correlation which can accurately predict the value of the forced convection heat transfer coefficient is also carried out. The research was carried out theoretically and numerically. Theoretically, thermodynamic calculations and analysis were carried out to determine the supercritical ORC performance of 10 organic fluids by varying the fluid pressure, mass flux and heat flux. Literature and theoretical studies were also carried out to determine the published Nusselt number correlations and can be used to calculate the HTC of organic fluids at supercritical pressures. Numerically, the HTC and its heat transfer characteristics were obtained through CFD simulation using the k - ???? turbulence model. The heat transfer process from thermal oil flowing in annulus of double-pipe counter flow HE types to propane flow in a 8 mm diameter pipe is modeled with a 2D model using a axisymmetric. HTC calculation results from this CFD simulation are used as data to formulate a new Nusselt number correlation using the curve fitting method. Furthermore, the HTC value calculated from the new Nusselt number correlation was validated using HTC data from CFD simulations that had been validated with experimental data in the form of graphs and validated from the results of calculations using other published correlations. The results obtained from the theoretical study are that for low-calorie energy sources, R-290 produces isentropic net work which is better than R-1270, R-134a, and R-227ea for both subcritical pressure, critical pressure, and supercritical pressure. its efficiency is still below R-134a. A new Nusselt number correlation that can predict the value of the convection heat transfer coefficient at supercritical pressure has been obtained in this study, this correlation has an estimated standard error value, and a correlation coefficient of Sy/x is 0.0333 and R is 0.9875 and has been validated using published correlation equations and CFD simulation data. The increase in supercritical pressure will reduce the increase and decrease in the forced convection heat transfer coefficient around the pseudo-critical temperature of propane while the increase in mass flux will increase the HTC. The increase in mass flux also reduces the temperature of the propane exiting the supercritical heat exchanger (SHE). The ratio of the mass flux of propane to the mass flux of thermal oil (as a heat/energy source) with the best maximum temperature of 150 oC is 1:5. Comparison of HTC values of the new correlation of the Nusselt number with R-134a and propane HTC at various supercritical pressures and mass flux has deviations between -30% to 20%.