SusTEM Special Sessions on Thermal Energy Management MODELING AND CFD ANALYSIS OF A MINIATURE RADIAL TURBINE FOR DISTRIBUTED POWER GENERATION SYSTEMS Kiyarash Rahbar, Saad Mahmoud, Raya K. Al-Dadah, Ahmed Elsayed School of Mechanical Engineering University of Birmingham
Introduction Steam/water Rankine cycle Vs organic Rankine cycle Importance of of expander Aims and objective Design methodology of radial turbo-expander Preliminary design Detailed design Results Preliminary design Detailed design Proposed specifications Conclusion
Introduction Accelerated world s energy consumption has led to scarcity of fuel resources and severe environmental pollutions New solutions and alternatives are required Distributed (on site) Power Generation (DPG) is a promising solution for supplying energy demands and reducing environmental problems DPG is an electric power source connected directly to the distribution network or the customer site of the meter Category Distributed Micro Power Generation Distributed Small Power Generation Distributed Medium Power Generation Distributed Large Power Generation Power Rating 1Watt to 5kW 5kW to 5MW 5MW to 50MW 50MW to 300MW Ackermann T et al.distributed Generation: a definition. J Electric Power Systems Research 2001;57:195 204
Steam/Water vs. Organic Rankine Cycle Water/steam Rankine cycle 1. Has uneconomically low thermal efficiency when exhaust steam temperature drops below 370ᵒC 2. Bulky equipments due to high specific volume of steam 3. High capital cost, safety concerns and complex system due to requirements of high temperature and pressure 4. High maintenance cost due to erosion and corrosion of blades caused by steam droplets 5. Unavailability of high temperature heat sources in DMPG Organic Rankine cycle 1. Suitable to be powered by low grade heat sources in temperature range of 60-200ᵒC 2. Small size due to high fluid density (Steam=2.4kg/m 3,R245fa=17.6 kg/m 3 at 5bar,200ᵒC) 3. Simplicity and alleviation of safety concerns due to low pressure and temperature 4. Low capital and maintenance cost due to use of non-eroding and noncorrosive working fluids 5. Availability of low grade heat sources when supplied by renewable energies
Importance Of Expander Key component of the DPG Plays a major role in determining the overall cycle efficiency 1. Velocity type: turbo expanders (Radial and Axial) 2. Displacement type: scroll, screw and reciprocal piston expanders
Radial Turbo Expander Radial turbo expanders offer many advantages over axial turbo expander and displacement type expanders Simple structure and easier manufacturing (one-piece casting) compare to axial turbo expander (blades and disk) Compact size due to greater specific power than equivalent axial stage (Euler turbomachinery equation) High efficiency Light weight
Aims And Objectives Design and CFD analysis of a small size radial turbine Applicable for distributed micro power generation systems with power capacity of 5kW Operating in organic Rankine cycle Suitable to be powered by low grade heat sources such as solar or geothermal energies in temperature range of 60-200ᵒC
Design Methodology Of The Radial Turbo-Expander Main goal is to minimize the losses and maximize the efficiency of turbine with following constraints Geometric Physical Economic This goal is accomplished by a systematic approach consisting of two main phases: Preliminary phase Detailed phase
Preliminary Design Determines the overall characteristics and the performance levels Highly iterative since it requires comprehensive trade studies of many different designs by variation of large group of input parameters 1-D code based on conservation of mass, momentum and energy and Euler turbo-machinery equation and appropriate loss models Mean streamline through the stage represents an average of the passage conditions at each key calculating station
Detailed Design Concentrates on 1 or small number of design candidates that offer the optimum combination of features based on preliminary design results Investigates the aerodynamics of the flow field with much greater accuracy CFD analysis employed using ANSYS CFX (full three-dimensional Reynolds- Average Navier-Stokes equations with appropriate turbulence modeling)
Results- Preliminary Design Table of variation range of input parameters Algorithm for systematic variation of input parameters Parameter Range Inlet Total Temperature( C) 60 200 Inlet Total Pressure (kpa) 150 400 Pressure Ratio 1.5 3 Mass flow rate (kg/sec) 0.03-0.1 Rotational speed (rpm) 40000 60000 Velocity Ratio 0.65-0.85 Inlet relative flow angle (degree) Exit absolute flow angle (degree) -60 - -15-10 10
Results- Design Space Design Space 84 Rotor total to total efficiency(%) 82 80 78 76 74 72 70 0.82 0.80 0.78 0.76 0.74 0.72 Velocity Ratio(U/C) 0.70 0.68 0.66 0.4 0.6 0.8 1.0 1.2 Specific Speed(Ns) Each point represent an individual turbine design with different operating conditions and geometry
Results- Preliminary Phase Variation of inlet total temperature and pressure Variation of mass flow rate and pressure ratio
Power(kW) Rotor total to static efficiency(%) Rotor inlet diameter(mm) Results- Preliminary Phase Variation of velocity ratio and rotational speed 5 Variation of rotor relative inlet and absolute exit flow angles 82 100 4 3 2 1 0-65 -55-45 -35-25 -15 Rotor relative inlet flow angle(deg) Alpha 2= 10 deg Alpha 2= 0 deg Alpha 2= -10 deg 80 78 76 74 72-65 -55-45 -35-25 -15 Alpha 2= 10 deg Alpha 2= 0 deg Alpha 2= - 10 deg Rotor relative inlet flow angle(deg) 80 60 40 20 0-65 -55-45 -35-25 -15 Rotor relative inlet flow angle(deg) Alpha 2= 10 deg Alpha 2= 0 deg Alpha 2= -10 deg
Results- Detailed Phase Parameter Selected Value Inlet Total 60 Temperature( C) Inlet Total Pressure 200 (kpa) Pressure Ratio 2 Mass flow rate (kg/sec) 0.09 Rotational speed (rpm) 55000 Velocity Ratio 0.685 3 different blade profiles were investigated with the aim of achieving appropriate blade loading and uniform flow The case with best blade profile was investigated for the appropriate number of rotor blades using CFD analysis Inlet relative flow angle (degree) Exit absolute flow angle (degree) -35 0
Results- Detailed Phase-Variation Of Blade Profile Total Power output= 3.871kW Case a Total Power output=3.834kw Case b Total Power output=3.791kw Case b
Results- Detailed Phase-Variation Of Rotor Blade Counts Z=8 Z=12 Z=15
Specifications Of The Proposed Radial Turbo-Expander Parameter Unit Value Power kw 4 Total to total isentropic efficiency % 85.3 Nozzle diameter at TE mm 94.8 Nozzle vane height mm 14.5 Nozzle throat area mm 2 253 Nozzle blade inlet angle to radial degree 0 Nozzle blade exit angle to radial degree 75 Nozzle blade number - 27 Rotor inlet diameter mm 82.5 Rotor exit diameter at tip mm 53.6 Rotor exit diameter at hub mm 24.7 Rotor blade inlet angle to radial degree 0 Rotor blade inlet angle to axial at degree -70 RMS Rotor blade number - 12
Conclusion There is a need for designing a small scale radial turbo expander for distributed micro power generation systems based on organic Rankine cycle Two techniques were employed as preliminary design phase and detailed design phase An algorithm was developed for the preliminary phase in order to explore a large number of designs based on a parametric study to determine the best initial design for the system of interest Preliminary design tool does not provide adequate information regarding to the complex 3-D behavior of the fluid inside the expander CFD analysis tool was also employed as the detailed design tool to investigate in greater details the characteristics of design candidate that was recommended by the preliminary phase Turbine with efficiency of 85.3%, power of 4kW and rotor diameter of 8cm is suitable to be used for supplying energy demands in DMPG systems
Closing Thanks for listening and patience Any questions?