A heat transfer model for heat exchanger was derived to investigate the effects of intermittent operations on the thermal performance of a heat exchanger with U shape buried pipes.In the model,the calculation area was divided into two parts,one in the borehole and the other outside the borehole,with the borehole wall as the boundary,a steady heat transfer model and a numerical unsteady heat transfer model were applied respectively.Three intermittent operation modes were defined as those of office building,shopping mall and residential building.The calculation results indicated that after one month operation,the heat fluxes under the three modes increased by 45.57%,58.43% and 88.05%,respectively,compared with a continuous operation mode,and the largest restoration of heat flux and average temperature were under the residential building mode.

A new model combing numerical simulation and analytical solution is proposed for U tube ground heat exchanger with varied heat flux boundary. The calculation area is divided into two parts by the boundary of the borehole wall. For the area in the borehole and the surrounding soil area, the calculation method is steady analytical solution and finite volume method respectively. In the borehole, steady heat transfer model is established based on energy conservation. During the calculation, the first variables should be obtained are the exiting fluid temperature and heat flux per unit length which are calculated by steady analytical method with the known borehole temperature at the start time, and then the borehole temperature and the surrounding soil temperature profile in the very time step are simultaneously calculated by solving the transient heat conduction equation with varied heat flux boundary, the following work is circularly calculating exiting fluid temperature, heat flux per length which was considered as the heat flux boundary for the next time step. The simulation agrees well with the corresponding experiment data which demonstrate the accuracy of the numerical simulation in present work.

Heat transfer of ground pipe heat exchanger for soil source heat pump system has been reviewed.Simulation method and long term operation performance of soil source heat pump system has been analyzed,the hybrid GSHP(HGGSHP) heat exchangers has also been introduced.Furthermore,the paper summarizes the shortcoming of the previous research work and the problem of the ground heat exchanger to be solved in the future work.

Lead-free piezoelectric ceramics(Bi0.5Na0.5)0.9Ba0.07Sr0.03TiO3(BNBST) doped with MnO2 were fabricated with the solid-state reaction technique.With Mn(x(MnO2) = 0-1.25%,mole fraction,the same below) doping,all the ceramics can be well sintered at 1 150 ℃ and exhibit a dense and pure perovskite structure.The X-ray diffraction patterns indicate that a morphotropic phase boundary(MPB) between tetragonal and rhombohedral phases exists at 0.20x0.75,and the Mn-doped BNBST ceramic with x = 0.25 exhibits excellent properties,where piezoelectric constant d33 = 157 pC/N,electromechanical coupling factor kp = 33%,mechanical quality factor Qm = 364,relative dielectric constant er = 843,and dielectric loss tan δ = 2%.These results confirm the relationship between the MPB composition and x and thus provide a method for designing new piezoelectric materials.

The effect of intermittent operation on the heat transfer performance of vertical U buried pipe heat exchangers is studied by method of numerical simulation and analytical solution. The calculation area is divided into two parts by the boundary of the borehole wall. For the area in the borehole, the calculation method is steady analytical solution, and for the soil area, the method is finite volume method. In the borehole, steady heat transfer model is developed based on energy conservation, the two parts are coupled by the temperature of the borehole. According the actual operation mode of Office Building, Living Building and Marketplace three intermittent operation modes are determined: 10 + 14(operates 10 and stops 14 hours in one day), 12 + 12 and 14 + 10. The results show that by combining the analytical and numerical method, the method in current work provides a rapid and accurate solution. Under continuous operation, the unit heat flux decreases and the average temperature of borehole wall increases. During the intermittent operation, the heat flux and average temperature of borehole wall fluctuates, the heat flux increases after each intermittence, and the average temperature of borehole wall decreases. The restoration of 10 + 14 model is the best due to the biggest intermittence ratio (the ratio of intermittence time and operation time). With the operation proceeding, the enhancement of the heat flux caused by the intermittence becomes more obvious, and simultaneously the restoration ratio of average temperature of borehole decreases.