چكيده به لاتين
In this thesis thermo-hydraulic modeling of flow in absorber tube of Parabolic Trough Collectors (PTC) with Direct Steam Generation (DSG) and heat transfer enhancement has been investigated. An appropriate Lumped model is used for evaluation of thermo-hydraulic process inside tube in order to identify critical condition of the system. Effect of direct normal irradiance (DNI), ranging from 200-875 on flow pattern and thermo-hydraulic characteristics including temperature, pressure, convection coefficient and void fraction of fluid along absorber tube were analyzed. The separation steam quality was assumed equal to 0.8. With decreasing DNI, the stratified flow pattern will be the dominant pattern in two phase region causing high temperature gradient and thermal stress in tube. In some cases this thermal stress cause the bending of receiver and deviates it from focusing line and even damages the receiver. A code was written in Engineering Equation Solver (EES) and the results were validated against experimental data from INDITEP project. good agreement was observed. The current modeling can be used as a preliminary design tool for a DSG loop. After identifying the critical operation condition, twisted tape was proposed for heat transfer enhancement and temperature difference decreasing in superheated region.
Afterwards, heat transfer enhancement by twisted tape in parabolic trough collector with superheated steam as a working fluid, was numerically studied. Numerical method is based on Finite volume method and for turbulence modeling RNG k-ε model was used. Numerical results were validated with experimental correlation and good agreement between numerical result and experimental correlation was observed.
The effect of twisted tape geometrical parameter, fluid inlet temperature and Reynolds number on the Nusselt number, friction factor and thermo- hydraulic performance were studied. Superheated steam temperature that was used in this simulation was 557, 598 and 632 kelvin.
Numerical computation was done by commercial Ansys 14 software with Reynolds number in the range 10000-978772, clearance ratio in the range 0-0.5 and twist ratio in the range 2.5-10 .
According to numerical results, by using twisted tape the maximum circumferential temperature gradient was decreased about 45 percent which reduce thermal stress and absorber tube bending. Nusselt number increase in Range 20- 200 times with increase in friction factor is in range 1.8-4.1.
