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The low thermal conductivity of phase change material (PCM) seriously weakens the heat charging and discharging rates of latent heat storage (LHS) system. High efficient performance enhancement method is urgently needed. In present study, three-dimensional simulation models with and without natural convection were established for a shell-and-tube LHS unit to investigate the effects of PCM arrangements and natural convection on the charging and discharging performance. The results show that compared ot the commonly used shell-and-tube LHS unit with PCM in shell side, the LHS unit with PCM in tube side can significantly enhance heat storage rate under the same working conditions and overall dimensions. Natural convection has significant effects on charging performance, espeically when PCM is arranged in tube side. When natural convection is neglected, PCM melting time can be reduced by 25.4% and latent heat storage rate can be enhanced by 36.6% with PCM arranged in tube side. When natural convection is considered, PCM melting time can be reduced by 34.4% and latent heat storage rate can be enhanced by 54.2% with PCM arranged in tube side. However, natural convection has little effects on discharging performance even if for the model with PCM in tube side.

In this study, two typical components in industrial flue gas (SiO 2 solid particle and H 2 O vapor) was selected as a solid-gas system, the transport properties were investigated by equilibrium molecular dynamics (EMD) simulation with three different concentrations of solid phase systems established over the actual temperature range (1000–1400 K) of industrial flue gas. The number of hydrogen bonds was counted and the radial distribution function in the system was obtained to analyze the microstructure of the system. The thermal conductivity, viscosity and diffusion coefficient were calculated and the effects of temperature and solid concentration on the transport properties were investigated. The predicted values of transport properties are in good agreement with the available experimental data. The results show that with the diffusion of water molecules, polar water molecules form hydrogen bonds with silica and other water molecules, and as the temperature increases, the average number of hydrogen bonds of water molecules decreases and the stability of the system decreases. Simultaneously, with the increase of temperature, the thermal conductivity and viscosity of the system increase, when the concentration of the solid phase in the system increases, both the thermal conductivity and the viscosity increase at the same temperature. This study improves the understanding of transport characteristics of industrial flue gas from the microscopic point of view, which is significant to the dust purification and waste heat recovery technologies.

Nanomaterials have been widely used to prepare nanocomposite phase change material (NPCM) and enhance its thermal conductivity. However, most of the studies are mainly focused on the thermal transport properties and the microscopic mechanism is still not clear. In this paper, molecular dynamics (MD) method was adopted to study both the thermal and phonon transport behaviors of NPCM (CuO/paraffin) to deeply reveal the influence mechanism of nanomaterials on PCM. The results show that the nanoparticle can enhance PCM thermal conductivity, and the enhancement effect is strengthened with nanoparticle mass fraction and NPCM temperature increasing. After that, phonon density of states was calculated to investigate the phonon scattering phenomenon in NPCM. The results show that the phonon scattering is very small in nanoparticle; then with the distance to nanoparticle surface increasing, the phonon scattering quickly increases; however, there exists a nanolayer around the nanoparticle, where the phonon scattering almost keeps constant. The weaker phonon scattering, the higher thermal transport performance. So, the existence of nanolayer caused by nanoparticle reduces the phonon scattering and promotes the heat transfer, which contributes to the thermal conductivity enhancement of NPCM

A compound enhancement method was proposed to improve the latent heat storage (LHS) performance of a shell-and-tube LHS unit, which is consisted of internal enhanced tube (ET) and multiple phase change material (PCM). Numerical validations on the presented method were performed based on comparisons of four different LHS cases: case 1 (basic case); case 2 (simple enhancement case); case 3 and 4 (compound enhancement cases). The results show that the simple enhancement case can only enhance PCM melting rate at the first half of the LHS tube; the compound enhancement case can obtain the synergy enhancement effect for the whole LHS tube. Compared with case 2, the PCM melting time is reduced by 37.3% and 17.4%, the total charging time is reduced by 25.6% and 16.9% for case 3 and case 4 respectively. For case 3, although the PCM melting time and charging time are the shortest, the total thermal energy storage (TES) capacity is reduced by 26.9% due to the lower PCM melting enthalpy. For case 4, not only the melting time and charging time can be obviously reduced, but also the total TES capacity is augmented by 6.6%.

In order to enhance the performance of the latent heat storage (LHS) process and provide the criterion for the selection and match of the multistage PCMs, the effects of PCMs melting temperatures on the heat storage rate, entransy dissipation rate and heat storage quality were numerically analyzed based on the entransy theory. For the single stage LHS unit, although decreasing the PCM melting temperature can augment the heat storage rate, the lower melting temperature causes larger entransy dissipation and reduces the heat storage quality. The larger heat storage rate results in the larger entransy dissipation rate, which is accordant with the entransy dissipation extremum theory. For the two-stage LHS unit with reasonably matching the PCMs melting temperature, the heat storage rate can be augmented and the entransy dissipation rate can be reduced. Then the optimization for the match of the two-stage PCMs melting temperatures was performed based on the entransy theory. The results show that there is an optimal match of the two-stage PCMs melting temperatures to achieve the maximum heat transfer rate or the minimum entransy dissipation rate. And the formulas for the optimum two-stage PCMs temperatures were presented, which can provide the criterions for the selection and match of the PCMs.

The solar energy flux distribution on the outer wall of the inner absorber tube of a parabolic solar collector receiver is calculated successfully by adopting the Monte Carlo Ray-Trace Method (MCRT Method). It is revealed that the non-uniformity of the solar energy flux distribution is very large. Three-dimensional numerical simulation of coupled heat transfer characteristics in the receiver tube is calculated and analyzed by combining the MCRT Method and the FLUENT software, in which the heat transfer fluid and physical model are Syltherm 800 liquid oil and LS2 parabolic solar collector from the testing experiment of Dudley et al., respectively. Temperature-dependent properties of the oil and thermal radiation between the inner absorber tube and the outer glass cover tube are also taken into account. Comparing with test results from three typical testing conditions, the average difference is within 2%. And then the mechanism of the coupled heat transfer in the receiver tube is further studied.

The air-side heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger with delta winglets are investigated numerically. The three-dimensional simulations are performed with renormalization-group (RNG) k − ɛ model to lay the foundation for the design of the high-performance heat exchanger. The wavy fin-and-tube heat exchangers which have three-row round tubes in staggered or in-line arrangements are studied. The numerical results show that each delta winglet generates a downstream main vortex and a corner vortex. For the in-line array, the longitudinal vortices enhance the heat transfer not only on the fin surface in the tube wake region but also on the tube surface downstream of the delta winglet; for the staggered array, longitudinal vortices are disrupted at the first wavy trough downstream from the delta winglet and only develop a short distance along the main-flow direction, and the vortices mainly enhance the heat transfer of the fin surface in the tube wake region. The longitudinal vortices generated by delta winglet cause considerable augmentation of heat transfer performance for wavy fin-and-tube heat exchanger with modest pressure drop penalty. When R e D c = 3000 , compared with the wavy fin, the j and f factors of the wavy fin with delta winglets in staggered and in-line arrays are increased by 13.1%, 7.0% and 15.4%, 10.5%, respectively.

The objective of this work is to train an artificial neural network (ANN) to predict the performance of gas cooler in carbon dioxide transcritical air-conditioning system. The designed ANN was trained by performance test data under varying conditions. The deviations between the ANN predicted and measured data are basically less than ±5%. The well-trained ANN is then used to predict the effects of the five input parameters individually. The predicted results show that for the heat transfer and CO2 pressure drop the most effective factor is the inlet air velocity, then come the inlet CO2 pressure and temperature. The inlet mass flow rate can enhance heat transfer with a much larger CO2 pressure drop penalty. The most unfavorable factor is the increase in the inlet air temperature, leading to the deterioration of heat transfer and severely increase in CO2 pressure drop.

Three dimensional numerical studies were performed for laminar heat transfer and fluid flow characteristics of wavy fin heat exchangers with elliptic/circular tubes by body-fitted coordinates system. The simulation results of circular tube were compared with the experiment data, then circular and elliptic ( e = b / a = 0.6) arrangements with the same minimum flow cross-sectional area were compared. A max relative heat transfer gain of up to 30% is observed in the elliptic arrangement, and corresponding friction factor only increased by about 10%. The effects of five factors on wavy fin and elliptic tube heat exchangers were examined: Reynolds number (based on the smaller ellipse axis, 500 ∼ 4000), eccentricity ( b / a , 0.6 ∼ 1.0), fin pitch ( F p /2 b , 0.05 ∼ 0.4), fin thickness ( F t /2 b , 0.006 ∼ 0.04) and tube spanwise pitch ( S 1 /2 b , 1.0 ∼ 2.0). The results show that with the increasing of Reynolds number and fin thickness, decreasing of the eccentricity and spanwise tube pitch, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. There is an optimum fin pitch ( F p /2 b = 0.1) for heat transfer, but friction factor always decreases with increase of fin pitch. And when F p /2 b is larger than 0.25, it has little effects on heat transfer and pressure drop. The results were also analyzed from the view point of field synergy principle. It was found that the effects of the five factors on the heat transfer performance can be well described by the field synergy principle.

Three-dimensional numerical simulations were performed for laminar flow of wavy fin-and-tube heat exchangers by using body-fitted coordinates (BFC) method with fin efficiency effect accounted. The prediction results of average Nusselt number, friction factor and fin efficiency were compared with the related experimental correlations [R.C. Xin, H.Z. Li, H.J. Kang, W. Li, W.Q. Tao, An experimental investigation on heat transfer and pressure drop characteristics of triangular wavy fin-and-tube heat exchanger surfaces, J. Xi'an Jiaotong Univ. 28 (2) (1994) 77–83] and Schmidt approximation [T.E. Schmidt, Heat transfer calculations for extended surfaces, Refrigerating Engineering (April 1949) 351–357]. For Reynolds numbers based on the tube outside diameter ranging from 500 to 4000, the mean deviation is 3.3% for Nusselt number, 1.9% for friction factor and 3.6% for fin efficiency. The distributions of local Nusselt number and fin efficiency on fin surface were studied at wavy angle equal to 0° (plain plate fin), 10° and 20° respectively. The local Nusselt number decreases along the air flow direction, but fin efficiency increases in general. The wavy angle can greatly affect the distributions of local Nusselt number and fin efficiency, and make the distributions present fluctuation along the flow direction. The result also shows that the fin efficiency at the inlet region of wavy fin is larger than that of plain plate fin at the same region. With the increase of Reynolds number, the effects of wavy angle on the distributions of local Nusselt number and fin efficiency are more and more significant.

In this paper, 3-D numerical simulations were performed for laminar heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger by body-fitted coordinates system. The effect of four factors were examined: Reynolds number, fin pitch, wavy angle and tube row number. The Reynolds number based on the tube diameter varied from 500 to 5000, the fin pitch from 0.4 to 5.2 mm, the wavy angle from 0° to 50°, and the tube row range from 1 to 4. The numerical results were compared with experiments and good agreement was obtained. The numerical results show that with the increasing of wavy angles, decreasing of the fin pitch and tube row number, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. The effects of the four factors were also analyzed from the view point of field synergy principle which says that the reduction of the intersection angle between velocity and fluid temperature gradient is the basic mechanism for enhance convective heat transfer. It is found that the effects of the four factors on the heat transfer performance of the wavy fin-and-tube exchangers can be well described by the field synergy principle.

Three-dimensional numerical studies were performed for the performance of wavy fin-and-tube heat exchangers in Body-Fitted Coordinates (BFC) system. Effects of geometric parameters on air-side heat transfer and fluid flow characteristics and fin efficiency were examined. The results showed that with the increase in Reynolds number, wavy angle, fin thickness and the decrease in fin pitch and transverse tube pitch, the heat transfer performance are enhanced; however, pressure drops are also increased. So, in practical applications, the wavy angle had better be located between 10° and 20° and the fin pitch should be located between 1.2 mm and 2.0 mm. The fin efficiency and average fin surface temperature decrease with the increase of Reynolds number, wavy angle, fin pitch and transverse tube pitch. With the increase of fin thickness, the fin efficiency and average temperature on fin surface also increase.