查看更多>>摘要：Gas turbine blade trailing edge cooling is challenging because of the stringent geometrical constraints driven by aerodynamics and thermal stresses。 The blade topology becomes significantly thin towards the trailing edge which leaves a narrow room for the construction of internal cooling channels and also makes this section susceptible to failure due to thermal stresses。 Conventionally， pin-fins are employed in the internal cooling channels of the trailing edge because they provide high levels of heat transfer coefficient values and also structural integrity。 Much of the past studies on pin fins have focused on their relative arrangements， heights， crossflow scheme， etc。， with an aim to enhance the endwall heat transfer coefficient as well as leverage the conjugate heat dissipation capabilities of these “extended surfaces”。 Lattices on the other hand， have been investigated to a lesser degree， and some studies can be found for blade mid-chord region， however， with their manufacturability challenges at that time， the concepts did not gain traction。 Lattices are far more complex topologies with superior local heat transfer characteristics as well as high conjugate heat dissipation capabilities due to large wetted surface area。 With the recent advancements in metal additive manufacturing， complex topologies such as lattices can be revisited due to high chances of their realization now。 To this end， we are presenting our study on lattices additively manufactured in 420 Stainless Steel (thermal conductivity nearly twice of Inconel 718)， where four different unit cell topologies， (a) Octet， (b) Tetrakaidecahedron， (c) Face diagonal-cube， and (d) Cube， were printed using Binder Jetting technology。 The aim of this study is to characterize the conjugate heat transfer capabilities of these lattices manufactured in 420 Stainless Steel prior to conduct similar study with Inconel 718， due to relatively simpler additive manufacturing route of Binder jetting (420 stainless steel) compared to Direct Metal Laser Sintering (Inconel 718)。 The intended design porosity of the samples was 0。886 and the coupons had single unit cell (edge size of 10 mm) in the thickness。 The steady-state experiments were conducted to evaluate the effective thermal conductivity and forced convective heat transfer performance。 For forced convection， the experiments were conducted for a wide range of Reynolds numbers with air as the working fluid。 Overall heat transfer coefficient， pressure drop， Nusselt number and friction factor enhancement level and pumping power requirements of these structures are presented。 The implications of the porosity variation due to manufacturing process on the interpretation of relative performance trends is discussed。 The effective thermal conductivity was independent of the topology for considered porosity value。 Face diagonal-cube yielded the highest heat transfer and pressure drop for the investigated Reynolds number range。 However， Cube provided the best overall thermal hydraulic performance value of 1。94 to 2。78 for the investigated range of Reynolds number。 At fixed pumping power conditions， Cube was capable of providing the highest heat transfer coefficient。
查看更多>>摘要：Cold storage technologies using clathrate hydrates as working media have recently attracted more attention in the energy storage field because they undergo phase transitions with large latent heats and suitable temperatures for cold storage。 However， the application of hydrate cold storage technologies is limited by the fact that the cold discharge mechanism is still unclear。 Thus， this paper adopted the method of external melting by internal heat exchange to investigate the heat transfer， energy conversion and efficiency of a novel tetrabutylammonium bromide (TBAB) hydrate cold storage system during cold discharge。 The system consisted of an internal circulating gas disturbance device and a metal spiral hydrate-on-coil heat exchanger。 The heat transfer mechanism and cold discharge capacity， rate， and efficiency for different cold charge temperatures， flow rates and disturbance modes were compared and analyzed。 A novel time-efficiency number was introduced to evaluate the comprehensive performance of the time， energy conversion and efficiency of cold discharge。 The results showed that the heat exchange balance during the cold charge and discharge stages had a thermal buffering effect on the storage unit。 Forced convection caused by gas disturbance resulted in a larger cold discharge capacity and reduced the discharge time by a factor of 3 compared to operation without gas disturbance。 In addition， the cold discharge capacity gradually increased with decreasing cold charge temperature and increasing flow rate。 The cold discharge efficiency was more than 83% under different conditions。 A comparison showed that the time-efficiency number tended to be larger with gas disturbance。 This study showed that the time-efficiency number can be used for quantitative comparison of the cold discharge time and efficiency in hydrate cold storage systems。
查看更多>>摘要：Helically coiled tube with smooth wall (HSW) has been widely used in the fields of supercritical air conditioning systems and supercritical heat pump systems due to its compact structure and excellent heat transfer performance。 However， its large circumferential non-uniformity and intense axial oscillation will bring to local stress concentration and instability of heat transfer which are both responsible for system accidents。 To alleviate the above two crises， the spiral inner wall roughness is proposed to be introduced into HSW。 Up to now， the performance of helically coiled tube with internally ribbed roughness (HRW) has been seldom reported and the effectivenesses of inner roughness with different structures on suppressing circumferential non-uniformity and axial oscillation are still unclear。 In this work， the SST k-ω model is employed to simulate the turbulent flow of s-R1234ze(E) in horizontal HRW to fill these gaps。 Firstly， the effectivenesses of different rib heights， rib lift angles， rib widths， rib numbers， and rib base angles on restraining circumferential non-uniformity and axial oscillation are explored。 The influences of wall thickness are also contained。 Subsequently， the internally ribbed roughness and concaved roughness are compared and evaluated according to the overall comprehensive performance criterion。 Finally， the effects of spiral roughness， centrifugal force， and gravitational buoyancy force on the flow field distributions are also revealed。
查看更多>>摘要：The pulse tube refrigerator (PTR) with passive displacer is a potential alternative for high-efficiency PTR。 One of the main problems is that massive experiments are expensive and time-consuming。 A back-propagation neural network (BPNN) model is proposed to overcome this challenge。 The cooling capacity and relative Carnot efficiency of the PTR with passive displacer can be predicted instead of experiments when the charging pressure， operating frequency， input electrical power， and cooling temperature are determined。 The optimal BPNN with the least mean square error is obtained by optimizing the network structure， transfer function， and training function。 As a result， the coefficient of determination (R2) values for the training and testing data are 0。9995 and 0。9992， respectively。 The predicted values are achieved within eight milliseconds， agreeing with the experimental values。 The maximum relative Carnot efficiency of 23。2% occurs at 140 K with a cooling capacity of 15 W。 It is concluded that the BPNN is an effective model for simulating the performance of the PTR with passive displacer quickly。 Furthermore， the mathematical formulas and the capability to predict the effects of the operating parameters are demonstrated， which is scarce in the literature on the PTR。 The BPNN model can be applied to accelerate the development of the PTR with passive displacer。
查看更多>>摘要：Liquid slugging， which often happens during startup and defrosting processes of the air conditioning， is a great threat to the reliability of rotary compressors， because of its high pressure in the compression chamber。 In this paper， a coupled mathematical model that can predict the compression process of refrigerants in three different states in the compressor cylinder is proposed。 The refrigerant phase change， heat exchange between refrigerant and wall， leakage through leak passage and refrigerant kinetic energy change rate are considered in the model。 The coupled solution method and its Fortran calculation program are carried out。 This mathematical model is experimentally verified by using R290 as refrigerant。 The results show that the simulation results of pressure variation in the compressor cylinder during the refrigerant two-phase compression and discharge processes are consistent with the experimental values。 The liquid slugging happens with the maximum pressure of 2400 kPa in the cylinder under low suction gas mass fraction。 The R290 is more prone to condense during in-cylinder compression than that of R22 and R410A under low wall temperature condition。 The larger the volume change rate of the compression chamber， the higher compressor rotation speed， and the smaller the flow area at the end of discharge process will exacerbate the liquid compression phenomenon。
查看更多>>摘要：The quality of modeling the thermal behavior of a battery also depends on the accuracy of the entropic heat coefficient (EHC) applied。 The EHC has so far been experimentally determined using time-consuming measuring methods。 This paper shows the application of a method determining the EHC based on experiments and numerical optimization。 The applied practical steps to determine the EHC are presented and discussed。 This method provides sufficiently accurate results for practical applications within a relatively short time (within six days)。 The simulations and measurements for the validation were performed simultaneously on a Hardware-in-the-Loop test bench。 At the beginning of each experiment， the characteristic maps for the battery module's electrical simulation were unknown and then determined simultaneously online。 The simulation results compared to the simultaneously recorded measurement data of different dynamic test scenarios (about one hour each) show a temperature difference of at most 0。4K， with an overall increase of 3。9K。 Furthermore， the results show that the influence of the EHC contributes significantly to the resulting temperature curve and should not be neglected in thermal analyses of a battery: Deviations of up to 38% in the temperature values have been shown when the EHC is not considered。
查看更多>>摘要：To realise a fast supply of hot water， this study proposes a compact latent heat exchanger。 This heat exchanger consists of a certain number of heat-exchange plates。 In each plate， serpentine tubes are wrapped with an inorganic composite phase change material (CPCM)， Ba(OH)2·8H2O/modified expanded graphite (MEG)。 An auxiliary electric heating film is attached to the face of the entire heat exchanger to improve the heating efficiency。 The parameters of the plate， such as thickness， tube pass number， thermo-physical properties of the CPCM， flow rate， auxiliary heating power size， and position are numerically evaluated to achieve the best performance of this heat exchanger。 The reliability of the simulation method is verified by comparing the simulated results with the experimental data in previous study。 The simulation results indicate that a plate with a thickness of 15 mm， plate tube pass number of 10， and CPCM with a Ba(OH)2·8H2O mass fraction of 85。8 wt% is the optimal choice。 Based on the optimal conditions， 196。48 kg of hot water over 40 ℃ can be produced in 875 s and the average power reaches 23 kW。 Furthermore， with the addition of a heating film (3600 W)， the mass of heated water can be increased by nearly 50 kg。
查看更多>>摘要：Frosting is frequently seen in nature and engineering fields。 To avoid the drawbacks and take advantage of frost， analyzing the frost characteristics is meaningful。 Frosting experiments on an inverted cold plate at surface temperatures of ?30 ～ -10 °C under natural convection are carried out。 Frost characteristics， including frosting stages， frost layer thickness and surface roughness， dynamic frosting rate and reverse melting， are analyzed and discussed。 Results show that stages of droplet condensation and coalescence， solidified liquid tip-growth and frost layer growth are within 220 s with their frost thickness less than 450 × 10-6 m。 Formation of large condensate droplets under influence of gravity causes a longer solidification time to 160。2 s and decreases the surface roughness。 As surface temperature decreases， frost thickness increases and surface roughness fluctuates more intensely， with its amplitude maximum at 102。29 × 10-6 m。 Dynamic frosting rates at different surface temperatures increase linearly and then declines in a curve， and their trend varies when first reverse melting occurs。 At the surface temperature of ?25 °C， amount and time of reverse melting are 9。42 × 10-8 m2 and 1。25 s， respectively， leading to the largest downward of dynamic frosting rate。 Contributions of this study are expected to provide a more in-depth understanding of frosting and thus provide a reference for optimal defrosting control in engineering applications。
查看更多>>摘要：In the present work， the effect of periodic Electrohydrodynamic (EHD) actuation modes on enhancing heat transfer and cutting down energy consumption in a channel is numerically investigated。 For this purpose， a simple duty-cycled voltage is applied to the wire。 Moreover， the performance of the periodic actuation is compared with the steady actuation。 A descriptive EHD efficiency is used to evaluate the effect of the periodic EHD actuation on heat transfer enhancement and electric power consumption。 The results show that reducing the time duration of the actuation in the periodic mode leads to reducing the friction factor， heat transfer efficiency， and energy consumption。 The results also show that using the periodic actuation for Reynolds numbers of 500 and 1000 compared to the steady actuation case increases the performance evaluation criterion and reduces energy consumption by 70% and 50%， respectively。 The results indicate that compared to the steady EHD actuation， the energy consumption of periodic EHD with the time duration of the actuation of 0。1 s is reduced by 90%， while the heat transfer efficiency decreases by 32。05% at Re = 500。 Consequently， the EHD efficiency increases by reducing the time duration of the actuation and the Reynolds number。 Furthermore， the results indicate that replacing one wire in steady actuation with two wires in periodic actuation improves heat transfer or decreases friction factor without changing the energy consumption。
查看更多>>摘要：We present high boiling heat transfer properties achieved by magnetically aligned nickel precursor inks on copper substrates measured atmospheric pressure。 Alignment was performed in a single direction and grid orientation。 Pool boiling studies were performed to obtain correlations between the heat flux， heat transfer coefficients， and wall superheat using water and ethanol。 The effects of surface wettability and roughness on nucleate boiling heat transfer properties and bubble dynamics are reported。 Our studies yielded a critical heat flux (CHF) of 185。9 W/cm2 and a heat transfer coefficient (HTC) of 106。9 kW/m2 °K for water on Ni grid patterned surface， representing an improvement of 49。9% in CHF and 105% in HTC compared to plain copper surface。 The alignment of nickel followed by its sintering introduced nucleation characteristics that improved bubble dynamics。 This is attributed to a) altered three-phase contact angle via chemically heterogeneity of Ni/Cu surface， and b) increased surface roughness due to nickel microstructures that induced wickability。
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Elsevier