Study Of Flow And Heat Transfer Performance Of Recuperator For Micro Gas Turbine

The distributed energy supply systems have attracted more and more attention in recent years with the adjustment of the energy structure and more attention paid by the government to environmental protection than before.And micro gas turbine as the power device plays an important role in distributed energy supply system.Recuperator is an important component of micro gas turbine and it plays a major role in improving the thermal efficiency of micro gas turbine.And as of now,the design,manufacture and use of micro gas turbine recuperator are mature in foreign countries,however,there are few public reports on this aspect due to the reasons of technical confidentiality.The research on the performance of high temperature recuperator is still in a relatively weak stage in China and there are few research data on this aspect,so it is of great significance to accelerate the development of efficient and compact recuperator for micro gas turbine.Based on the above problems,this paper designs a plate recuperator for 300kW micro gas turbine.And the hydraulic and entropy production characteristics of the annular primary surface recuperator are studied.An experimental plate recuperator was manufactured and its fluid flow and heat transfer characteristics were tested in recuperator performance-testing platform.Furthermore,based on the test data in experimental research,the numerical simulation was conducted to analyze the design and off-design performances of recuperator.And then the problem of unbalanced flow distribution in counter current heating exchanging region at the low temperature side was solved by changing the length of the rib on the condition of design requirement.The thermodynamic model of micro gas turbine was established,and the influence of recuperator performance on the overall performance of micro gas turbine is studied.The whole plate recuperator model was established to analyze the static and dynamic performance.For the annular primary surface recuperator,the flow and heat transfer and entropy generation characteristics in cross-wavy?CW?channels are analysed by means of numerical computation.In this paper,according to the given parameters of recuperator for 300kW micro gas turbine and the empirical correlation for air flowing in rectangular channel,a plate recuperator which consist of hot and cold side plates has developed.And the tested sample which contain partial hot and cold side plates was machined,welded and assembled.The flow heat transfer performances of plate recuperator were studied by experimenting under some working conditions.Based on the experimental results,the hot and cold side flow resistance and heat exchange efficiency of counter current heating exchanging region of plate recuperator at design and off-design conditions were studied by numerical simulation in detail.The results show that hot and cold side pressure lose is 1.1%and 2.87%in design conditions,respectively.And the heat exchange efficiency of plate recuperator is greater than 0.86,which meets the design requirements of recuperator.In addition,the heat exchange efficiency of plate recuperator has little relation with the inlet temperature of hot and cold side,but it has close relation with the inlet flow rate of hot and cold side.In addition,the overall structure of the plate recuperator is optimized,and the channel width and length of the cold side diversion zone are reasonably designed so that the cold fluid can be evenly distributed to sub-regions when it transferres from the narrow inlet diversion zone to the large-width counter-current heat transfer zone,which ensures the higher recuperator's compactness and the uniformity of outlet temperature.Further detailed numerical study shows that the flow distribution of the cold fluid is not uniform among the channels in the single sub-region of counter-current heat transfer zone at cold side,which makes the exit temperature of the channels in the single sub-region of counter-current heat transfer zone uneven.In order to solve this problem,a structural optimization scheme of increasing the rib height of each channel at the entrance of cold side sub-region is proposed.By improving the structure,the average flow velocity of each channel in the single subregion of counter-current heat transfer zone varies from4 to 6m/s,and the distribution is relatively uniform.By calculating,it is found that the maximum temperature difference between the outlet temperature of the hot side channel is reduced by 5?,the average outlet temperature is reduced by 1.7?after improving the structure,and the average outlet temperature of the cold side channel is increased by 2.2?after improving the structure.Under the condition of allowed resistance loss,this optimization scheme can effectively improve the heat transfer performance of the counter-current heat transfer zone.The model of whole plate recuperator is established to study the static and dynamic performance on the Flownex SE software platform.The results show that when the inlet flow and pressure on hot and cold side remain unchanged,the flow resistance of hot side increases slightly with the hot side inlet temperature increases,and the cold side flow resistance and recuperator heat efficiency changes small.When the mass flow rate and pressure of hot and cold side remain unchanged,the hot side flow resistance is basically unchanged and the cold side flow resistance is reduced and the recuperator heat efficiency is basically unchanged with the increase of the cold side inlet pressure.When the inlet temperature and pressure of hot and cold remain unchanged,the flow resistance of hot and cold sides increases with the mass flow rate of hot and cold side increasing at the same time,and the recuperator heat efficiency decreases slightly.For the step change of the inlet temperature of hot side,the response time of the outlet temperature of cold side is significantly faster than that of the hot side.For the step rise of 60?of hot inlet temperature,it takes 200s for the cold side to return to the stable state,and 500s for the hot side to return to the stable state.However,when the mass flow rate of hot and cold side decreases by the same amount,the time of hot and cold outlet temperature restoring stability is similar.At the same time,the smaller the step change is,the shorter time it takes for the temperature of both sides to recover and stabilize.In addition,the thermodynamic model of 300kW micro gas turbine was established on the Flownex SE software platform,and the model of the plate recuperator is substituted into the thermal cycle of the 300kW micro gas turbine.Under the condition that the compressor and turbine speed remain unchanged,the thermal cycle efficiency increases with the increase of the output power,and the recuperator heat transfer efficiency decreases with the increase of the output power.Furthermore,when the hot and cold side pressure loss increase,the output power and efficiency of micro gas turbine decrease,and the change of cold side pressure loss has a greater impact on the output power and efficiency than the change of hot side pressure loss.It is significant to design and optimize the micro gas turbine control system.For the primary surface heat exchanger,the effects of the ratio of channel amplitude to wavelength?A/L?and channel height to pitch?H/P?of cross-wavy?CW?channels on thermal-hydraulic performance are investigated numerically based on its periodic structure and multi-period boundary division method.The results show that the heat transfer is enhanced with increasing A/L value at the same H/P.Meanwhile friction factor increases with increasing A/L.Similarly,increase of H/P makes the heat transfer enhance at the same A/L.However,the friction factor is almost unchanged with increasing H/P value.In addition,the numerical study to research the influence of ratio of pitch to radius of air channel?P/R₂?and height to radius of air channel?H/R₂?on entropy generation are conducted.The simulation results show the thermal entropy generation accounts for a large part of the total entropy generation.And dimensionless thermal entropy generation rate in gas channel has increased by 85.2%at Re=1020 with P/R₂ value ranging from 6.0-7.2 and it has decreased by 62.5%at Re=1020 with H/R₂value ranging from 8.8-12.8.For another,dimensionless thermal entropy generation rate in air channel has increased by 28.5%at Re=1148 with P/R₂ value ranging from6.0-7.2 and it has decreased by 68.9%at Re=1148 with H/R₂ value ranging from 8.8-12.8.The conclusion can provide reference for the design and optimization of CW primary recuperator.