Research On Heat Transfer Enhancement And Deposition Mechanism Of Exhaust Heat Exchanger

Engine is the most important powerplant in transportation industry,but the average brake thermal efficiency of ICEs remains at 30%-40%.As an effective method for heat recovery and efficiency improvement,waste heat recovery(WHR)system has attracted the attention of many scholars.Exhaust heat exchanger,as a key component,has a significant effect on the efficiency of WHR system.Its heat transfer,weight and deposition performance can directly affect the thermal efficiency,practicability and long-term performance of WHR system,respectively.This paper investigates the heat transfer enhancement and deposition mechanism for exhaust heat exchanger in WHR system,four exhaust heat exchangers with different structure types are considered.Then,an enhanced exhaust heat exchanger with compact design is performed based on the metal foam material.Besides,the deposition performance of performed heat exchanger is analyzed and optimized in this paper.The main conclusions of this work are listed as follows:(1)Experimental test on the performance of double-pipe heat exchanger(DPHE).Taking into account the engine WHR system,the exhaust heat exchanger should be capable of working for long time in high-temperature,high-pressure and corrosive conditions.Besides,gas pressure drop is also severely restricted due to the effect of exhaust back-pressure of engine.Thus,as the most basic structure in shell and tube heat exchanger,a DPHE is designed and manufactured to recover residual heat in the exhaust gas.The experimental results show that the DPHE can recover the heat in exhaust gas effectively.Besides,the result of multi-parameter(heat transfer,pressure drop and weight)performance evaluation shows that the power loss caused by weight is significant,thus a compact design is required to improve the comprehensive performance of exhaust heat exchanger.(2)Research on thermal-hydraulic characteristics and compact potential of DPHE.The results show that the gas pressure drop at the corners and near-wall region of the shell side is more obvious,and the metal foam material wrapped on the tube side can greatly enhance the heat transfer performance.Therefore,by reducing the number of heat exchanger rows,the metal-foam wrapped shell and tube heat exchanger(MW-STHE)shows similar thermal-hydraulic performance with almost only 1/3 the volume and weight of the conventional DPHE.This result shows that metal foam has great potential for compact design.Besides,MW-STHE with Ni-0610 metal foam has the best comprehensive performance among all considered samples.(3)Design and performance comparison of metal-foam baffle shell and tube heat exchanger(MF-BCHE).Based on the internal intercommunicable structure of the metal foam material,a novel MF-BCHE is performed to improve the performance of exhaust heat exchanger.The pressure loss and stagnant zones can be reduced effectively in the MF-BCHE because part of the exhaust gas can flow through the baffles.Thus,compared with DPHE and traditional metal-baffle shell and tube heat exchanger(MB-STHE),the performance of MF-BCHE under different gas flow rate(0.01-0.07 kg/s)and foam thickness(3-9 mm)can be increased up to 178.93% and583.61%,respectively.Moreover,all four heat exchangers considered in this research have been evaluated under the same structure and working condition.The results show that MF-BCHE shows the best comprehensive performance.(4)A high-precision and self-designed experimental bench is established for performance test of heat exchanger under exhaust particle flow.Firstly,the hightemperature air with specified mass flow rate and temperature is provided by a combination of vortex blower and air heater.Then,the soot particles with specified parameters produced by soot generator are mixed into the high-temperature air to form a simulated exhaust.Besides,the uncertainty of the experimental bench is analyzed and the evaluation method of change regularities of deposition performance is proposed.This experimental bench lays the foundation for the research on the deposition mechanism of MF-BCHE.(5)Performance test of MF-BCHE under the work condition of high-temperature particle flow.Based on the established experimental bench,the deposition performance of MF-BCHE is tested and analyzed from two perspectives of flow style(cross-flow over tube banks)and heat transfer material(metal foam).Change regularities of tube banks and partial filled metal foam channel are tested under different work conditions.The results show that the anti-fouling performance of channel with partial filled metal foam is better.The performance deterioration of partially filled metal foam channel after 6 h of deposition is about 1.44%,the performance deterioration of tube bank after 4 h of deposition can reach up to 10.38%.(6)Establishment of deposition prediction model and optimization of deposition performance of tube banks.A new prediction model for soot deposition in the recuperative heat exchanger under the work conditions of exhaust particle flow is proposed in this paper.Besides,the two-dimensional model of tube banks is established,and its deposition performance under the flow style of cross-flow over tube banks is analyzed and optimized based on the prediction model.The results show that the soot particles on the tube bank are mainly deposited around ±30° and ±150°.When the tube spacing is 1.25,the tube bank shows the best heat transfer performance and anti-fouling performance;when the tube spacing is 2.0,the tube bank shows the best pressure drop performance and comprehensive performance.