Evaluation of Helium Xenon Gas Mixture as Working Fluid in Highly Loaded Axial Compressor*

Adil Malik Qun Zheng Hamza Fawzy Naseem Ahmed Salman A.Ahmed

（College of Power and Energy Engineering,Harbin Engineering University,Harbin China,adilmalik@hrbeu.edu.cn）

Abstract：This paper evaluates the use of helium xenon binary gas mixture having molecular weight of 15 g/mole as working fluid in an axial compressor of terrestrial nuclear power plants as working fluid. Unadulterated helium is one of the best coolant because of its prevalent transport properties, so it is hard to compress. Its utilization in high temperature gas cooled reactor (HTGR) vitality change framework lead to greater size, increasingly mass, greater expense and dynamic issues of turbomachines. In this paper a highly loaded helium xenon compressor is designed and its performance analysis is conducted. It is observed that, only 18 % stages of those in helium compressor are required to compressed the gas for desired pressure in highly loaded helium xenon compressor. The compressor of high temperature gas cooled reactor (HTGR) working on Closed Brayton cycle (CBC) reduced to 2 against 16 stages. Thus,in turbo compressors ofHTGRpower plants, use of helium xenon over pure helium is advantageous.

Keywords：Highly Loaded Design,Helium Xenon Compressor,Closed Brayton Cycle(CBC),High Temperature Gas Cooled Reactor(HTGR),Helium Compressor

0 Introduction

In this era,a lot of researchers are making efforts to obtain more reliable,efficient and stable energy supply with continual use of high temperature gas cooled reactors working on closed Brayton cycle,since it supplies a huge amount of energy with negligible amount of CO2emission during lifetime.High temperature gas cooled reactor(HTGR)is a 4th generation nuclear reactor with contemporary potential of security,low-cost and ecological safeties[1-5].Numerous High temperature gas cooled reactor(HTGR)designs are being studied for electricity generation at optimum thermal efficiency of greater than 48%,for supply of heat in many energy conversion processes[6-9].HTGR plants having helium as coolant with indirect and direct Closed Brayton Cycles(CBCs)are being analyzed for energy conversion in USA,Europe,Russia,Japan and South Africa.Systematic diagram of HTGR plants use in direct and indirect CBCs for energy conversion are shown in Fig.1.The reactor coolant is the CBC working fluid in a direct CBC system.In indirect CBC system working fluid could be the similar or dissimilar from reactor coolant.

Pure helium is the best coolant due to its superior transport properties.Helium compressor is one of major component in HTGR energy conversion system as it compresses helium i.e.working fluid of HTGR reactor at desired operating conditions.Helium is less compressible gas than air and requires more number of stages in order to attain requisite pressure ratio.These unfavorable conditions lead to multistage narrow flow path,which increases the aerodynamic losses linked with separation losses,boundary layer losses,tip clearance losses,friction losses and mismatching of rotor stator stages.Moreover,narrow compressor rotors create dynamic problems[10-11].Therefore,there is a dire need to resolve this issue.

In order to resolve this problem,researchers put forward a concept of highly loaded compressor design method focusing on thermophysical properties of helium[12].The researchers increased the flow coefficient in order to reduce the number of stages[13].Based on this research a highly loaded design method for compressors was developed[14].Various fluid flow phenomenon in highly loaded axial compressor cascade are being studied experimentally and numerically for optimum designing of highly loaded compressors with minimum losses and to control separation characteristics[15-21].On the other hand,a lot of research is in progress for identifying optimum working fluid for HTGR system with better electrical output performance,thermal effi-ciency,smaller size,less mass and minimum dynamic losses.The shorter shafts of double spools turbomachinery in HTGR power plants are easy to maintained,offer excellent stiffness and have better dynamic performance[22],and increases the overall plant efficiency in comparison with longer shafts.

Fig.1 Systematic diagram of HTGR plants

It is prudent to highlight that present experience of designing helium turbomachinery and operating a closed Brayton cycle is very limited and immature and nearly non-existent with noble gases and its binary mixtures as a working fluid.Actually,selection of the gas as a working fluid for HTGR energy conversion system working on direct CBC depends on its availability,compatibility and neutronics.Noble gases and its binary mixture gases such as helium nitrogen,helium neon,helium argon and helium xenon are being researched for its use as working fluid in HTGR power plants based on closed Brayton cycle and axial flow turbomachinery[23-25].Experimental investigation studies on heat transfer of helium xenon mixtures in triangular channels[26],in the cylindrical channels[27],in the initial pipe sections[28]and in the heated channels with different cross-sectional shapes[29]are being conducted with studies on the calculation of transport coefficients and thermodynamic properties of helium xenon gas mixture[30].Helium xenon mixture binary gas mixture was also used in project Prometheus space nuclear power plant,which was a direct coupling of Brayton energy conversion loop(s)with a single reactor heat source through the gas coolant/working fluid[31].

The mixture of helium xenon having 15 g/mole molecular weight has maximum heat transfer coefficient that is 7%higher heat transfer rate as compared to pure helium at requisite pressure and temperature.Therefore,in this paper highly loaded helium xenon compressor is designed for a binary gas mixture of helium xenon having 15 g/mole molecular weight as working fluid,to be used in gas cooled reactor(HTGR)energy conversion system and its performance was analyzed.Effect of He-Xe mixture on blade loading,compressor performance curves,Mach number and estimated stall margin are analyzed in order to eliminate the problem of higher number of stages causing aerodynamic losses,rotor dynamics problems and helium leakage failures in helium compressor.

1 Highly Loaded Helium Xenon Compressor

As no experimental data of highly loaded helium xenon compressor is available,so a highly loaded helium xenon compressor has to be derived from available empirical relations of air or helium compressor.Velocity vectors and respective velocity diagram of a highly loaded axial flow compressor are shown in Fig2.The velocity triangle of highly loaded compressor marginally increases the torsional velocity with the increase in both the axial velocity and a negative prewhirl.Thus,in case the circumferential velocity is constant,stage loading is marginally increased.

It is decided to utilize high reaction blade in order to achieve both the design objectives of higher efficiency and broader surge margin in a minimum number of compressor stages.CDA profile as shown in Fig.3 is chosen for the helium xenon compressor as the flow is subsonic in the cascade.Subsequently,incidence is adjusted to get one stagnation point near to leading edge.On the suction side,maximum Mach number point is restricted and suction side diffusion till trailing edge is achieved without separation with the“Stratford”velocity profile of minimum skin friction.

Fig.2 Velocity diagram of highly loaded compressor

Furthermore,bowed stator vane is designed with optimum angle in order to eliminate suction side corner separation as show in Fig.4 and to maintain incidence of flow close to end walls 3D stacking is used.A helium xenon compressor first stage was designed with one row of rotor and stator based on a validation case compressor of a 300 MW HTGR-GT power plant.The selection of working fluid influences the turbo compressor mainly in two particular aspects.Firstly,it affects the stage number required to attain requisite pressure ratio and efficiency.Secondly,it effects the machine dimensions required for a high pressure CBC system.Helium xenon compressor rotational speed is chosen as 3600 RPM because of generator requirements.In dynamic machines blade speed dictates the size of machine.With a constant load factor,increasing the circumferential speed marginally increases the work done at each stage with reduction in number of stages as the loading factor is inversely proportional to the square of the blade speed.Reducing the number of stages caused by increasing the peripheral speed is obvious,so it is encouraged to select the maximum value proportionate with stress restriction in order to get minimum number of stages.

Fig.4 Bowed stator vane for highly loaded helium xenon compressor

In order to reduce the helium xenon binary mixture compressor stages,utilizing mixture sonic velocity in comparison with air,it was decided to select flow coefficient of 0.96 for highly loaded helium xenon compressor.In a low speed compressor,stage loading and reaction are to be decreased or increased with reference to each other,in order to achieve optimum efficiency.Stage loading and reaction must be selected with respect to each other to successfully increase the efficiency irrespective of flow coefficient[32].Thus,reaction of highly loaded helium xenon compressor is selected as 0.7 with a loading factor of 0.54.The aerodynamic and physical parameters of design highly loaded helium xenon compressor is listed at Table 1.

Tab.1 Aerodynamic and physical parameters of design highly loaded helium xenon compressor

2 Performance Analysis of Highly Loaded Helium Xenon Compressor

2.1 Computational method

A helium xenon compressor 1st stage was designed using sate of the art highly loaded blade design technique based on Japanese 300 MW low pressure helium compressor(Reference Compressor)presented by Muto and Ishiyama.Numerical simulation of highly loaded helium xenon compressor 1st stage was performed using Ansys CFX.The computational model was exported to Numeca Turbo Grid for meshing and mesh was generated.The value ofy+was controlled within 5 in order to accomplish turbulence model requirements.A fine structure mesh of a single blade passage is generated with total grid number of more than 700,000 nodes.It was numerical simulated with He-Xe mixture,keeping the simulation procedure identical as used in validation case.Required He-Xe properties were calculated and requisite material was created in simulation.The properties of helium xenon mixture having molecular weight 15 g/mole is enumerated below in Table 2:

2.2 Comparison of blade loading distribution

The camber line of the helium xenon compressor and thickness distribution curves are generated through quadratic polynomials.In order to satisfy turning angle compulsions of blade,maximum deflection is controlled with the max aero foil thickness in chord percentage and location of maximum camber in tenths of chord during design process.Pressure distributions or blade loading diagram of reference compressor and designed helium xenon compressor 1st stage rotor and stator surfaces are shown in Fig.5.The area surrounded by the pressure lines represents the load of the blade.It can be seen that rotor and stator blade loading has drastically increased in 1st stage rotor and stator of the helium xenon compressor.The difference in pressure of suction side and pressure side is increased.Resultantly,loading increased and higher pressure ratio is achieved in helium xenon compressor.Blade loading has been increased due to decrease in value of specific heat at constant pressure with the addition of xenon in helium from 5 194 to 1 387J/kg·k.Subsequently,temperature rise per stage is increased.

Tab.2 Properties of He-Xe having molecular weight 15g/mole

2.3 Comparison of Mach number

Sonic velocity of helium is very high i.e 3 times more than air.However,with the addition of xenon in helium,the sound velocity of the helium xenon binary gas mixture having 15g/mole molecular weight is reduced to almost 1/2 of the helium.i.e.from 1 043.29to538.74m/s.Resultantly,Mach number is increased in cascade of helium xenon compressor as shown in Fig.6.Due to increase in Mach number the flow losses,profile losses and separation losses also increased.

2.4 Comparison of stage pressure ratio

The cost and size of axial flow turbomachines in HTGR power plant working on CBC increases with the increasing in number of stages for a certain operating pressure and temperatures.A large compressor requires more blades and disks,which increases the size and mass of the turbomachines and cause dynamic problems including rotor stator mismatch and bearing failures.Conversely,for the same rotational speed,reduction in number of stages will drastically reduces dynamic problems and cost and size of the turbo-machines.Fig 7 shows the stage pressure ratio of both the compressors i.e.reference compressor and designed helium xenon compressor having design point of both the compressors at equivalent inlet volume flowrate of 46 m3/s.Stage pressure ratio of reference compressor is very low however designed helium xenon compressor have very high pressure ratio.The single stage pressure ratio of helium xenon compressor is 1.209 which is marginally high as compared to single stage pressure ratio of 1.039 in reference compressor.Hence,the compressor stages of high temperature gas cooled reactor(HTGR)working on Closed Brayton cycle(CBC)reduced to 2 against 16 stages with the use of highly loaded helium xenon compressor.Overall LP and HP compressor stages i.e.35 stages can be reduced to 7 stages of single highly loaded helium xenon compressor.(in case no limitation on CBC cycle temperature).Thus,single spool turbo machine can be used to compress the gas up to requisite pressure.

Fig.5 Distribution of(a)rotor and(b)stator blade loading in reference and highly loaded helium xenon compressor

The selection of working fluid influences the turbo compressor mainly in two particular aspects.Firstly,it affects the stage number required to attain requisite pressure ratio and efficiency.Secondly,it effects the machine dimensions required for a high pressure CBC system.Helium specific heat at a constant pressure is five times more than air.Since,change in temperature rise in a compressor stage is inversely proportional to the specific heat at constant pressure(for a constant blade speed).Therefore,change in temperature rise available per stage will be only one-fifth to that of air while operating with helium as working fluid.It results in more number of stages required for compressor.However,in case of Helium xenon binary mixture having 15 g/mole molecular weight,the specific heat at constant pressure is reduced 4 times that of helium.Therefore,pressure ratio per stage is increased and will result in lesser number of stages for helium xenon compressor.

Fig.7 Stage pressure ratio in reference and highly loaded helium xenon compressor

2.5 Performance curves

Performance curves of compressor is very important in order to study the effect of modification or working fluid.Fig.8 shows the stage pressure ratio and isentropic efficiency of helium xenon compressor at various corrected mass flow rates and various rpms.Higher isentropic efficiency up to 93%is achieved with a pressure ratio of 1.209 at a designed mass flow rate of 630 kg/sec.Helium xenon compressor can operate over a wide range of flow rate at higher efficiencies.Estimated stall margin can be calculated as:

Estimated stall margin of design helium xenon compressor is 17.04%,which is well within designed limits.However,it is observed that stall margin of helium xenon compressor has been reduced in comparison with helium compressor.With the addition of xenon in pure helium,sonic speed of helium xenon mixture is reduced toof helium.Therefore,Mach number in helium xenon compressor is increased.This increase in Mach number has reduced the broader operation phenomenon in helium xenon compressor upto some extent as compared with helium compressor.In closed Brayton cycle plant power output is regulated through change in quantity of the working fluid in the system i.e.it is not necessary to change the power output by changing the speed of the compressor.Therefore,the reduction in surge margin of compressors in closed Brayton cycle systems are not as important as that inAero engines or other gas turbine power plants.

Fig.8 (a)Pressure ratio and(b)efficiency curves of highly loaded helium xenon compressor

3 Conclusion

At present,helium is favorable choice of working fluid.However,due to its lower molecular weight,higher number of stages are required to compress it,which result in more mass,bigger size,higher cost and dynamic problems of turbomachines.This study identified that the binary mixture of helium and xenon with molecular weight of 15g/mole is a better working fluid for high temperature gas cooled(HT-GR)nuclear power plants and its axial compressor.A highly loaded helium xenon compressor is designed and its performance is analyzed in order to study effect of helium xenon binary gas mixture thermo physical properties.Results shows that helium xenon binary gas mixture having 15g/mole molecular weight has the maximum heat transfer coefficient that is 7%higher heat transfer rate as compared to pure helium at requisite pressure and temperature.In addition,blade loading has marginally increased in a highly loaded helium xenon compressor resultantly higher pressure ratio has been achieved,which lead to reduction in number of stages i.e.mass,size and cost of compressor will reduce.Stall margin of highly loaded helium xenon compressor is reduced,however it will not effect the performance of compressor in CBC cycle.Hence,the use of helium xenon mixture having 15 g/mole molecular weight in power conversion unit of HTGR has the capability to eliminate the problem of higher number of stages causing aerodynamic losses,rotor dynamics problems and helium leakage failures due to its better transport and thermophysical properties.