Performance and Optimization of Air Source Heat Pump Water Heater with Cyclic Heating

A new type of microchannel condenser applied in the air source heat pump water heater(ASHPWH)with cyclic heating was proposed in this study.The operating performance of the ASHPWH was frst tested.Then,the structure of the microchannel condenser was optimized with the implement of vortex generators.Finally,a numerical model of the ASHPWH was established and the optimized microchannel condenser was studied.The experimental results showed that the average coefficient of performance(COP)of the 1HP(735 W)ASHPWH reached 3.48.In addition,the optimized microchannel condenser could be matched with a 3 HP(2430W)ASHPWH with an average heating capacity of 10.30 kW,and achieving an average COP of 4.24,14.6%higher than the limit value in the national standard.

Formation of fine fully-lamellar microstructure of TiAl-based alloy in rapid heating cyclic heat treatment process

The optical observation results of neocrystallization nucleation and growth of fine fully lamellar (FFL) α 2/ γ microstructure of a TiAl based alloy in rapid heating cyclic heat treatment process were reported. The characteristics of α+γ→α transformation under rapid heating conditions were analysed. A model for explaining the nucleation and growth mechanism of FFL α 2/ γ microstructure was proposed.

Effect of cyclic heat treatment on microstructures and mechanical properties of directionally solidified Ti-46Al-6Nb alloy

The Ti-46A1-6Nb （mole fraction, %） ingots that were directionally solidified by cold crucible were cyclic heat treated at 1330 ℃ in the a phase region. The microstructures and mechanical properties of the ingots before and after heat treatment were investigated. The results show that the large columnar grains are changed into equiaxed grains after heat treatment. The grain size decreases with increasing the cyclic times, which is caused by the recrystallization and the transition from the large grain of small lamellae to the small grain of large lamellae. Four times of cyclic heat treatment refines the grain size from 1.33 mm to 0.59 turn, nevertheless the lamellar spacing increases from 0.71 ~tm to 1.38 lim. Extending the holding time and increasing the cyclic times of heat treatment eliminate the fl-segregation at the grain boundary and the interlamellar. The compression testing shows that the compressive strength of the directionally solidified ingot in the parallel and perpendicular directions are 1385.09 MPa and 1267.79 MPa, respectively, which are improved to 1449.75 MPa and 1527.76 MPa after two and four times of cyclic heat treatment, respectively, while that is 1180.64 MPa for the as-cast sample. The fracture mode of the sample after cyclic heat treatment is quasi-cleavage fracture.

Effect of cyclic heat treatment on microstructure and mechanical properties of 50CrV4 steel

An annealed 50 Cr V4 steel was subjected to cyclic heat treatment process that consists of repeated short-duration(200 s)held at 840 °C(above Ac3 temperature of 790 °C) and short-duration(100 s) held at 700 °C(below Ac1 temperature of 710 °C). The spheroidization ratio of cementite and the average size of particles increase with increasing the cyclic number of heat treatment. After5-cycle heat treatment, the spheroidization ratio of cementite is 100%, and the average size of the cementite particles is about0.53 μm. After cyclic heat treatment, the hardness, ultimate tensile strength and yield strength of the experimental steel gradually decrease with increasing cyclic number of heat treatment. The elongation of the as-received specimens is about 7.4%, the elongation of the 1-cycle specimen is 14.3%, and the elongation of 5-cycle specimen reaches a peak value of 22.5%, thereafter marginally decreases to 18.3% after 6-cycle heat treatment. Accordingly, the fractured surface initially exhibits the regions of wavy lamellar fracture. By increasing the cyclic number of heat treatment cycles, the regions of dimples consume the entire fractured surface gradually. Some large dimples can be found in the fracture surface of the specimen subjected to six heat treatment cycles.

Strength-toughness improvement of 13Cr4NiMo martensitic stainless steel with thermal cyclic heat treatment

To improve the strength-toughness of 13Cr4NiMo martensitic stainless steel(13-4MSS),a thermal cyclic heat treatment(TCHT)combined with the advantage of tempering was proposed.The microstructures were characterized by scanning electron microscopy,X-ray diffraction and electron backscattered diffraction,and the mechanical behaviors in terms of tensile properties and impact toughness were analyzed in correlation with microstructural evolution.It was found that grains and the martensitic matrix were refined by TCHT through the cyclic quenching transformation and austenite recrystallization,which was conducive to more nucleation quantity of reversed austenite during tempering.Two-sphericalcap nucleation model was used to explain the effect of refined grains of TCHT on the nucleation of reversed austenite.Grain refinement by TCHT improved the brittle fracture stress to reduce the ductile-brittle transition temperature and thus improved the cryogenic impact toughness of 13-4MSS.Reversed austenite distributed at the martensitic lath boundary enhances the crack arrest performance and increases the britle fracture stress.It is concluded that reasonable TCHT plus tempering process significantly improves the strength-toughness of 13-4MSS,reflecting the comprehensive effect of grain refinement and reversed austenite.

Effect of cyclic heat treatment on abnormal grain growth in Fe-Mn-Al-based shape memory alloys with different Ni contents

Cyclic heat treatment that can continuously promote abnormal grain growth is widely used for the prepa-ration of single-crystal Fe-Mn-Al-based shape memory alloys.However,it takes a long time to prepare large-size Fe-Mn-Al-based alloy single crystals via the reported cyclic heat treatments.Meanwhile,the long-time cyclic heat treatment at high temperatures leads to the development of defects including oxidation and a decrease in Mn,which would deteriorate superelasticity in the Fe-Mn-Al-based shape memory alloys.To shorten the fabrication time of single crystals,the effect of the cyclic heat treatment process on the abnormal grain growth in the Fe-Mn-Al-based alloys with different Ni contents was systematically investigated.It is found that the abnormal grain growth of Fe-Mn-Al-based alloys was not significantly affected by the Ni contents(within 2.1 at.%-6.2 at.%).In addition,the abnormal grain growth could be promoted by 1-2℃ min^(-1) cooling rate,high solution temperature,and multiple cycles,while it was insensitive to other processes including heating rate,dual-phase time as well as long-time solution treat-ment.These findings can guide optimizing the fabrication process of single crystals by cyclic heat treat-ment.Finally,the Fe_(41.9)Mn_(37.8)Al_(14.1) Ni_(6.2) single crystal prepared by the optimized cyclic heat treatment showed a recoverable strain of about 4%.

Experimental Study on Transient Natural Convection in a Cube Enclosure with an Isolated Vertical Cyclically Heated Plate

The low frequency cyclical transient natural convection in a cube enclosure with an internal isolated vertical heated plate was investigated experimentally. A computer-aided experimental system was designed to generate the cyclical heating power input and also used for data reduction. The effects of the cyclic heating power input amplitude (from 0 to 8 W) and frequency (from 1/5400 s-1 to 1/600 s-1)as well as the per-cycle time-average power input (from 8 to 24 W) on the transient and time-average Nusselt number were parametrically studied. It was found that for such cyclical transient natural convection with low frequency, the plate heating power input amplitude and frequency have little effects on the time-average Nusselt number as long as the cyclical time-average heating power input remains the same, although the transient Nusselt number may be significantly affected. Therefore, the modified Grashof number based on the plate average heat flux can be used to characterize the time-average heat transfer process. The plate time-average Nusselt number is about 15% less than the infinite-space Nusselt number. The location of the isolated plate in enclosure does not appreciably influence the time-average heat transfer characteristics of the plate.

Precipitation kinetics of carbides during cyclical and isothermal aging of 2.25Cr–1Mo steel and its effect on mechanical properties

The carbide precipitation was analyzed during aging of 2.25Cr–1Mo steel at 550℃.The as-received steel was aged in two different manners:the first treatment was an isothermal aging at 550℃ for time up to 1000 h,and the second one was carried out by a cyclical aging,which consisted of aging at 550℃ for 1 h,water quenching at room temperature and then newly heating at 550℃.This procedure was repeated up to 300 times.The M_(23)C_(6) and M6C carbide precipitation took place intragranularly and intergranularly for both aging treatments.The carbide coarsening was detected to occur with the increasing aging time.Nevertheless,the growth kinetics of precipitation occurred more rapidly in the case of cyclical aging.The specimen hardness decreased with the aging time in both cases;however,it occurred in shorter aging time for the cyclical aging.Nanoindentation testing indicated the increase in ductility with the aging time,and the cyclically aged specimens showed larger ductility than the isothermally aged specimens.

Thermal-structural analysis of regeneratively-cooled thrust chamber wall in reusable LOX/Methane rocket engines

To predict the thermal and structural responses of the thrust chamber wall under cyclic work,a 3-D fluid-structural coupling computational methodology is developed.The thermal and mechanical loads are determined by a validated 3-D finite volume fluid-thermal coupling computational method.With the specified loads,the nonlinear thermal-structural finite element analysis is applied to obtaining the 3-D thermal and structural responses.The Chaboche nonlinear kinematic hardening model calibrated by experimental data is adopted to predict the cyclic plastic behavior of the inner wall.The methodology is further applied to the thrust chamber of LOX/Methane rocket engines.The results show that both the maximum temperature at hot run phase and the maximum circumferential residual strain of the inner wall appear at the convergent part of the chamber.Structural analysis for multiple work cycles reveals that the failure of the inner wall may be controlled by the low-cycle fatigue when the Chaboche model parameter c3= 0,and the damage caused by the thermal-mechanical ratcheting of the inner wall cannot be ignored when c3〉 0.The results of sensitivity analysis indicate that mechanical loads have a strong influence on the strains in the inner wall.