Study On Heat Transfer Performance And Heating Efficiency Of Electric Heater With Continuous Helical Baffles For Oil Shale In-situ Conversion

With the rapid development of China economy,there is a huge imbalance between China's oil and gas resource consumption and production.China is rich in oil shale resources.Heating it to the pyrolysis temperature(450-550 ?)can generate oil shale oil,which is a potential substitute for oil.In-situ conversion of oil shale is to convert oil shale into oil shale oil underground,which is an inevitable trend for clean and efficient development of oil shale.Temperature(heat)is a key factor in the in-situ conversion of oil shale.The downhole heating technology places the heater downhole,and the high temperature heat carrier produced can directly heats the oil shale layer.Downhole heater is the key apparatus of downhole heating technology.And it can be seen that the existing combustion downhole heaters have poor combustion stability and are not easy to reignite after the flame is extinguished.In addition,the existing electric downhole heaters heat the oil shale layer by heat conduction,which has low heating efficiency and there is no enhanced heat transfer structure on the heating rod surface,and the heater has a short lifespan.Therefore,this paper introduces the continuous helical baffle structure into the downhole electric heater for oil shale in-situ conversion,and uses the electric heating rod as the heat source to generate high-temperature air.In addition,a detailed study on improving the heat transfer performance and heating efficiency of downhole electric heater with continuous helical baffle is carried out.First,in order to study the influence of the helical pitch on the heat transfer performance and heating efficiency of the heater,numerical simulation and experimental research were carried out.Numerical analysis of the shell side flow field and temperature field of the heaters with segemental baffle and continuous helical baffle shows that the shell side flow of the heater with segemental baffle is turbulent,while the the shell side flow of heater with continuous helical baffle is uniform and the surface temperature of the heating rod is evenly distributed,so the continuous helical baffle structure is more suitable for downhole electric heaters for insitu oil shale conversion.Through the downhole electric heater experiment system,four continuous spiral baffle type downhole electric heaters with pitches of 50 mm,110 mm,160 mm and 210 mm(H50?H110?H160?H210)were tested.The results show that the greater the pitch of the continuous helical baffle,the higher the average temperature of the electric heating rod surface temperature,and the greater of its change rate,and the H50 is 36.8%-44.4%lower than H210.The influence of heating power and mass flow rate on the average surface temperature of electric heating rod is more significant.The comprehensive performance index K/?P does not change significantly with the heating power,but decreases significantly with the increase of the mass flow rate.Under the same conditions,as the pitch of the helical baffle decreases,the enhanced heat transfer capability of the heater increases,and the modified entropy production number and the dimensionless thermal resistance gradually decrease.H110,H160 and H210 have higher heating efficiency in the low Reynolds number range,while H50 has higher heating efficiency in the high Reynolds number range.Then,the influence of the packer location on the heat transfer performance and heating efficiency of the downhole electric heater was studied.Through the downhole experiment of the heater prototype,it is found that only when the head of the heat injection well is sealed,the high temperature air in the heat injection well will transfer heat to the top plate of the oil shale,and at the same time,local cold and heat convection will be formed at the bottom of the heat injection well.Therefore,it is proposed that the downhole electric heater and the packer work together to improve the energy utilization rate of high-temperature air.Based on this,a downhole operating condition simulation device was designed,and experiments were carried out on two schemes: a packer at the outlet of the heater(scheme 1)and a packer at the inlet of the heater(scheme 2).The results show that the outlet temperature of the heater rises rapidly in the initial stage,but gradually decreases in the second stage,and remains stable in the final stage.The duration of each stage is related to the pitch,heating power and mass flow rate.The shell side air temperature in Scheme 2 is higher than Scheme 1,therefore,the surface temperature of the electric heating rods of all downhole heaters in Scheme 2 is higher than that of Scheme 1.Except for H50,the comprehensive indicators show no obvious trend with the changes of mass flow rate and heating power.The total cost increases slowly and then sharply with the increase of the mass flow rate,and increases linearly with the heating power.In Scheme 1,the enhanced heat transfer capability of the heater is stronger,so its irreversible loss is less than that in Scheme 2.In the region of high Reynolds number,compared with H160 and H210,H50 has the smallest irreversible loss and the highest heating efficiency.In terms of the heat transfer performance,economy and heating efficiency of the heater,setting the packer at the outlet of the downhole electric heater is the best solution for downhole heat injection technology for in-situ oil shale conversion.Finally,in order to further improve the heating efficiency of the downhole electric heater,a double shell structure is proposed.By changing the air flow path of the shell side,the heat loss generated by the heater shell is recovered and reduced,thereby improving the heating efficiency of the electric heater.A comparative experiment was carried out on three-pitch single-shell and double-shell heaters to study the influence of the shell structure on the heater performance.The experimental results show that the influence of mass flow rate on heating rate is greater than heating power.In the fully developed section of the shell side flow field,the surface temperature of the electric heating rod increases linearly along the X axis.Except for the shell temperature at the inlet and outlet of the heater,the shell temperature of the countercurrent double shell heater(CDS-DEH)and the downstream double shell heater(PDSDEH)are 22.55%?80.00% and 55.94%?74.43% lower than that of the single shell heater(SSDEH),respectively.The air in the shell side absorbs heat from the electric heating rod surface in two ways: forced convection and thermal radiation,and mainly in forced convection heat transfer.The heater shell dissipates heat in two ways: thermal radiation and natural convection,and mainly in radiation heat transfer.The mass flow rate and heating power have no significant effect on the heating efficiency of the double-shell downhole electric heater,while the singleshell downhole electric heater has a significant effect.Within the scope of the experiment,the energy utilization rates of PDS-DEH,CDS-DEH and SS-DEH are 98.69%?99.80%,98.08%?99.65%,and 84.43%?94.25%,respectively.This indicates that the double shell structure can effectively improve the heating efficiency of the downhole electric heater by changing the air flow path in the shell side.The research conclusions of this paper can provide theoretical and technical guidance for China oil shale the in-situ conversion technology,especially the design of downhole heat injection scheme and the selection of heat injection process parameters.