Heat-transfer characteristics of a latent heat storage system using MgCl2 · 6H2O

Heat-transfer characteristics have been determined for the circular finned and unfinned-tube units during the freezing of magnesium chloride hexahydrate (MgCl₂ · 6H₂O) used as a phase-change material (PCM) with a melting temperature of 116.7 °C. The effects on the heat-transfer characteristics have been determined of the inlet temperature and the flow rate of air used as the heat-transfer fluid (HTF). With the unfinned-tube unit, the heat-transfer coefficients obtained between the PCM and the tube are larger than the calculated values based on the theory of steady-state heat conduction due to the dendritical crystal growth of PCM. The ratio of the heat-transfer coefficient of the finned-to the unfinned-tube systems is about 3.5 within the finned section and decreases gradually far from the finned section with an increase in crystal volume. The total amounts of heat recovered have been correlated in terms of the Fourier, Stefan, and Reynolds numbers to provide basic design data for circular finned- and unfinned-tube heat-storage units.     

Investigation of heat transfer during melting of a PCM by a U‐shaped heat source

An experimental study was conducted to investigate the melting process of a phase change material (PCM) and the associated convection heat transfer due to a U‐shaped heat source embedded in the PCM. The experiments were conducted at four input heat fluxes that varied from 3450 to 5840 W/m². The results showed that the heat transfer behavior, interface movement, and the heat transfer coefficients differed both axially and vertically inside the chamber. The local convective heat transfer coefficient in the inner region, enclosed by the U‐tube, was found to be about 35% higher than that in the outer region over the input heat flux range, resulting in faster melting in the inner region than in the outer region. As melted domain grew vertically from 15% to 100%, it was observed that the overall h in the inner region increased by 40–55% from the lowest to highest heat flux. The melting rate was also found comparatively high up until 65–70% of the total PCM volume melted because of the higher contribution from the inner region. It was also observed that the Rayleigh number increased by approximately 23% in the inner region and 18% in the whole domain as the heat flux increased from 3450 to 5840 W/m². A new Nusselt–Rayleigh number correlation is proposed for the heat transfer during the melting process due to a U‐shaped heat source. Copyright © 2017 John Wiley & Sons, Ltd.     

Improvement of heat transfer with perforated circular holes in finned tubes of air-cooled heat exchanger

The present study investigated the effect of perforated circular finned-tube (PCFT) on the convective heat transfer performance of circular finned-tube heat exchangers. The air-side convective heat transfer coefficients increased by 3.55% and 3.31% for 2-hole and 4-hole PCFT cases, respectively. The increase in the convective heat transfer coefficient was related to the reduction of the recirculation region by introducing the perforations at the flow-separation locations on the finned tube. The pressure drop across the finned-tube bundles increased by 0.68% and 2.08% for the 2-hole PCFT and 4-hole PCFT cases, respectively. The greater pressure drop in the case of the 4-hole PCFT might be due to excessive flow disturbances produced by multiple perforations. The fin factor defined as the ratio of the % increase of the convective heat transfer coefficient and that of the pressure drop was 5.19 for the 2-hole PCFT case, whereas that was 1.59 for the 4-hole PCFT case.     

Effects of particle sizes on methanol steam reforming for hydrogen production in a reactor heated by waste heat

This paper reports the effects of particle sizes on methanol steam reforming for hydrogen production in a reactor heated by waste heat. The unsteady model was set up, which has been applied to investigate the effects of particle sizes (1.77 mm–14.60 mm) on particle temperature, heat transfer quantity, overall coefficient of heat-transfer, etc. The heat transfer performance of waste heat recovery heat exchanger is improved when the particle size increases, which is conducive to increase hydrogen production. The particle temperature change rate, the specific enthalpy change rate, the moving velocity of the maximum heat release rate particle, the contribution rate of solid phases, the heat release rate and the overall coefficient of heat-transfer increase, but the effective time of heat transfer decreases. When the particle size increases from 1.77 mm to 14.60 mm, the solid phase average contribution rate increases from 89.43% to 94.03%, the overall coefficient of heat-transfer increases from 1.39 W m⁻² K⁻¹ to 13.41 W m⁻² K⁻¹, the heat release rate increases from 48.9% to 99.9% and the effective time of heat transfer reduces from 48 h to 6.7 h.     

Performance Analysis of Flat-Plate Solar Collector Having Silver Nanofluid as a Working Fluid

A study on water solar collector performance having silver nanofluid as working fluid was carried out. In this study, 20-nm silver particles mixed with water at the concentrations of 1,000 and 10,000 ppm were undertaken in 3 small identical closed-loop flat-plate solar collectors, each with an area of 0.15 m × 1.0 m. The mass flux of the working fluid varied between 0.8 and 1.2 L/min-m² and the inlet temperatures were controlled in the range of 35–65°C. The tests were performed outdoor under a steady-state condition. The experimental results showed that at the same Reynolds number, the convective heat transfer coefficient of the nanofluid inside the solar absorber tube at 1,000 ppm was slightly higher than that of water, and at 10,000 ppm, the heat transfer coefficient was about 2 times that of water. This meant that the overall heat loss coefficient of the solar collector with nanofluid could be reduced and more solar heat gain could be obtained, especially with a high inlet temperature of the working fluid. In our experiments, for 10,000 ppm concentration of silver nanoparticles, the optical characteristic and the thermal loss characteristic of the solar collector, under steady-state condition with a mass flux of 1.2 kg/min-m², were 0.691 and 4.869 W/m²-K, compared with 0.684 and 7.178 W/m²-K, respectively for 1,000 ppm concentration and 0.702 and 8.318 W/m²-K for water. When the flow rate was different from the standard value, the solar thermal characteristics were also improved with the nanofluid.     

An experimental study in determining the local heat transfer coefficients for the plate finned-tube heat exchangers

A three-dimensional inverse problem in determining the local heat transfer coefficients for the plate finned-tube heat exchangers utilizing the steepest descent method (SDM) and a general purpose commercial code CFX4.4 is applied successfully in the present study based on the measured temperature distributions on fin surface by infrared thermography.Two different tube arrangements (i.e. in-line and staggered) with different fin pitch and air velocity are considered and the corresponding local heat transfer coefficients are to be determined. Results show that some interesting phenomena of the local heat transfer coefficients for the finned surface are found in the work and the averaged heat transfer coefficient of the staggered configuration is about 8–13% higher than that of the in-line configuration.     

Falling film evaporation heat transfer of water/salt mixtures from roll-worked enhanced tubes and tube bundle

An experimental study is carried out for enhancement of falling film evaporation heat transfer of pure water and water/salt mixtures on horizontal smooth tube and two kinds of structured tube bundles under atmospheric pressure. The experimental results show that the low-cost roll-worked tube can greatly enhance the evaporation heat transfer performance of the falling film, and make it comparable to that of expensive commercial enhanced tubes such as GEWA-T tubes, TE tubes and HF tubes, even at low and moderate heat flux levels. The average evaporation heat transfer coefficients for the roll-worked tube bundle are basically independent from the parameters tested such as flow and heating conditions, salt-concentrations, as well as geometries of the tube bundles. The present experimental data result in a constant heat transfer coefficient; α≈20 kW/m² K, in the convective heat transfer range of the heat fluxes <10⁵ W/m².     

Experimental evaluation of energy and exergy efficiency of a seasonal latent heat storage system for greenhouse heating

In the following work, a seasonal thermal energy storage using paraffin wax as a PCM with the latent heat storage technique was attempted to heat the greenhouse of 180 m² floor area. The system consists mainly of five units: (1) flat plate solar air collectors (as heat collection unit), (2) latent heat storage (LHS) unit, (3) experimental greenhouse, (4) heat transfer unit and (5) data acquisition unit. The external heat collection unit consisted of 27 m² of south facing solar air heaters mounted at a 55° tilt angle. The diameter and the total volume of the steel tank used as the latent heat storage unit were 1.7 m and 11.6 m³, respectively. The LHS unit was filled with 6000 kg of paraffin, equivalent to 33.33 kg of PCM per square meter of the greenhouse ground surface area. Energy and exergy analyses were applied in order to evaluate the system efficiency. The rate of heat transferred in the LHS unit ranged from 1.22 to 2.63 kW, whereas the rate of heat stored in the LHS unit was in the range of 0.65–2.1 kW. The average daily rate of thermal exergy transferred and stored in the LHS unit were 111.2 W and 79.9 W, respectively. During the experimental period, it was found that the average net energy and exergy efficiencies were 40.4% and 4.2%, respectively. The effect of the temperature difference of the heat transfer fluid at the inlet and outlet of the LHS unit on the computed values of the energy and exergy efficiency is evaluated during the charging period.     

Hydrogen storage system based on hydride materials incorporating a helical-coil heat exchanger

Metal hydrides offer the potential to store hydrogen at modest pressures and temperatures with high volumetric efficiencies. The process of charging hydrogen into a metal powder to form the hydride is exothermic. The heat released by the reaction must be removed quickly in order to maintain a rapid charging rate. An effective method for heat removal is to embed a heat exchanger within the metal hydride bed. Here, we investigate the effectiveness of a helical coil heat exchanger tube to remove the heat generated during the absorption process. This paper presents a three-dimensional mathematical model formulated in Ansys Fluent 12.1 to evaluate the transient heat and mass transfer in a cylindrical metal hydride tank embedded with a helical-coil cooling tube. We present results from a parametric study of hydrogen storage efficiency as a function of helical coil pitch and convective heat transfer coefficient (h) within the cooling tube. We also explore the effect of adding aluminum foam to enhance the thermal conductivity of the metal hydride. The parametric study reveals that the mass of stored hydrogen is less sensitive to the coil pitch when aluminum foam is added. It is also found that the absorption rate increases with h as expected, although the rate of improvement diminishes at high values of h. Results were examined at filling times of 3 and 6 min to draw conclusions about the overall effectiveness of this hydrogen storage system. At 3 min, it is found that the addition of 5% Al foam is optimal, and h = 1000 W/m²-K is sufficient to bring the metal hydride to saturation; under these conditions a non-dimensional pitch of 0.5 maximizes the hydrogen absorption. Adding Al foam beyond 5% does not improve volumetric efficiency as the Al foam begins to displace the active hydrogen-absorbing material.     

Saturated flow boiling heat transfer and critical heat flux in small horizontal flattened tubes

This paper presents experimental results for flow boiling heat transfer coefficient and critical heat flux (CHF) in small flattened tubes. The tested flattened tubes have the same equivalent internal diameter of 2.2 mm, but different aspect height/width ratios (H/W) of ¼, ½, 2 and 4. The experimental data were compared against results for circular tubes using R134a and R245fa as working fluids at a nominal saturation temperature of 31 °C. For mass velocities higher than 200 kg/m²s, the flattened and circular tubes presented similar heat transfer coefficients. Such a behavior is related to the fact that stratification effects are negligible under conditions of higher mass velocities. Heat transfer correlations from the literature, usually developed using only circular-channel experimental data, predicted the flattened tube results for mass velocities higher than 200 kg/m²s with mean absolute error lower than 20% using the equivalent diameter to account for the geometry effect. Similarly, the critical heat flux results were found to be independent of the tube aspect ratio when the same equivalent length was kept. Equivalent length is a new parameter which takes into account the channel heat transfer area. The CHF correlations for round tubes predicted the flattened tube data relatively well when using the equivalent diameter and length. Furthermore, a new proposed CHF correlation predicted the present flattened tube data with a mean absolute error of 5%.     

Prediction of heat transfer coefficient on the fin inside one-tube plate finned-tube heat exchangers

The finite difference method in conjunction with the least-squares scheme and the experimental temperature data is proposed to predict the average heat transfer coefficient and the fin efficiency on the fin inside one-tube plate finned-tube heat exchangers for various air speeds and the temperature difference between the ambient temperature and the tube temperature. Previous works showed that the heat transfer coefficient on this rectangular fin is very non-uniform. Thus the whole plate fin is divided into several sub-fin regions in order to predict the average heat transfer coefficient and the fin efficiency on the fin from the knowledge of the fin temperature recordings at several selected measurement locations. The results show that the surface heat flux and the heat transfer coefficient on the upstream region of the fin can be markedly higher than those on the downstream region. The fin temperature distributions depart from the ideal isothermal situation and the fin temperature decreases more rapidly away from the circular center, when the frontal air speed increases. The average heat transfer coefficient on the fin increases with the air speed and the temperature difference between the ambient temperature and the tube temperature. This implies that the effect of the temperature difference between the tube temperature and the ambient temperature is not negligent.     

Productivity amelioration of stepped solar still using phase change material and evacuated tube collector

This study investigates the individual and cumulative effects of phase change material (PCM) and evacuated tube collector (ETC) on the performance of a stepped solar still (SSS). Experiments have been performed on SSS, SSS loaded with PCM (SSS-PCM), SSS coupled with ETC (SSS-ETC), and SSS loaded with PCM and coupled with ETC (SSS-PCM-ETC). An innovative way of loading paraffin wax as PCM is introduced to utilize solar energy efficiently for the distillation process during off-sunshine hours. ETC is used to provide pre-heated RO wastewater to the distillation unit. The distillate output was observed maximum for SSS-PCM-ETC (4.97 kg/m² day) which is 99% more than that of the SSS unit. The PCM and ETC individually with SSS unit increases the distillate output by 31% and 24%, respectively. The working time of the distillation unit was observed to be increased by 3 h by the use of PCM. The total heat transfer coefficient was evaluated at the maximum for SSS-PCM-ETC and was observed in the range of 16.94–167.04 W/m² °C. The thermal energy efficiencies of SSS, SSS-ETC, SSS-PCM, and SSS-PCM-ETC were evaluated as 28.65%, 35.59%, 43.88%, and 44.04%, respectively. SSS-PCM-ETC is found to be the most economic with the best environmental conservation having maximum values of daily productive cost ($0.69) and carbon credits earned ($184.8).     

A study of heat transfer in a circulating fluidized bed

An experimental investigation was carried out to study the effects of operating parameters on the local bed-to-wall heat transfer coefficient in a 4.5 m tall, 0.150 m diameter circulating fluidized bed with a bed temperature in the range of 65°C to 80°C, riser flow rate varying from 1400 litres/min to 2000 litres/min, bed inventory in the range of 15 kg to 25 kg of sand, and average sand sizes of 200 μm, 400 μm and 500 μm. A heat flux probe was attached to the riser wall at five different vertical locations for measuring the heat flux from the bed to the wall surface. From the present work, the heat transfer coefficient in the dilute phase was found to be in the range of 62 to 83 W/m²K, 51 to 74 W/m²K, and 50 to 59 W/m² K for sand sizes of 200 μm, 400 μm and 500 μm, respectively. Relevant mathematical correlations were developed to predict local heat transfer coefficient based on the results of the practical work.     

Heat transfer characteristics in a small-scale fluidized bed boiler

Heat transfer characteristics in a small-scale fluidized bed boiler (2MW_th) were studied using lignite and corn cob as fuels. Depending on air velocity, the heat transfer rates from bed to water membrane wall and from hot flue gas to convective tube bank were in the ranges 75–55% and 25–45% of the total heat absorbed by the boiler, respectively. At designed capacity, the heat transfer flux based on bed cross sectional area and on water membrane wall area were about 0·45 and 0·15 MWm⁻², respectively. Under the conditions studied, it was found that the overall heat transfer coefficient between bed and water membrane wall was 100–300 W m⁻² K⁻¹, whereas that between flue gas and convective tube bank was 10–30 Wm⁻² K⁻¹. The study of heat transfer to a horizontal tube immersed in the bed as well as placed in the freeboard region were also studied. The effective heat transfer coefficients were found to be 300–800 W m⁻² K⁻¹ for in-bed tube and 30–150 W m⁻² K⁻¹ for the freeboard region, depending on air velocity. Comparison of these data with those predicted by both modelling and correlation reported in the literature was also made. For the immersed tube, good agreement was observed for low air velocity, while at high air velocity the experiment produced results twice those estimated from modelling and correlation. For the freeboard region, the model gave a fair prediction.     

Local heat transfer coefficient for pool boiling of R-134a and R-123 on smooth and enhanced tubes

The current paper presents experimental investigation of nucleate pool boiling of R-134a and R-123 on enhanced and smooth tubes. The enhanced tubes used were TBIIHP and TBIILP for R-134a and R-123, respectively. Pool boiling data were taken for smooth and enhanced tubes in a single tube test section. Data were taken at a saturation temperature of 4.44 °C. Each test tube had an outside diameter of 19.05 mm and a length of 1 m. The test section was water heated with an insert in the water passage. The insert allowed measurement of local water temperatures down the length of the test tube. Utilizing this instrumentation, local heat transfer coefficients were determined at five locations along the test tube. The heat flux range was 2.5–157.5 kW/m² for the TBIIHP tube and 3.1–73.2 kW/m² for the TBIILP tube. The resulting heat transfer coefficient range was 4146–23255 W/m². °C and 5331–25950 W/m². °C for both tubes, respectively. For smooth tube testing, the heat flux ranges were 7.3–130.7 kW/m² and 7.5–60.7 kW/m² for R-134a and R-123, respectively; with resulting heat transfer coefficient ranges of 1798.9–11,379 W/m². °C and 535.4–3181.8 W/m². °C. The study provided one of the widest heat flux ranges ever examined for these types of tubes and showed significant structure to the pool boiling curve that had not been traditionally observed. Additionally, this paper presented an investigation of enhanced tubes pool boiling models.     

扁管通道的轿车暖风器传热性能研究

对轿车暖风器的传热性能进行了数值模拟和实验研究,将数值模拟与试验结果相对比,验证了壁面温度选取的正确,为进一步的数值模拟研究打下了基础。依据实测数据,将管内外侧的对流换热系数从传热系数中分离出来,得到了管外换热关系无量纲准则式;对管内外的换热特性、阻力特性进行分析,指出了改善暖风器传热性能的方法,同时验证额定工况性能指标的合理性。     

A comparison of flow boiling heat-transfer in in-line mini pin fin and plane channel flows

The use of a boiling fluid as a coolant is an attractive option for electronic devices as electrical power densities increase. However, for systems working at the micro-scale, design methods developed for evaluating heat transfer in macro-scale evaporators are not appropriate for passages with hydraulic diameter of the order of 1 mm and below.Heat-transfer coefficients and pressure drops are reported for two surfaces, a pin-fin and a plate surface, each with 50 mm square base area. The pin-fin surface comprised of 1 mm square pin fins that were 1 mm high and located on a 2 mm square pitch array covering the base. The channel was 1 mm high and had a glass top plate. The data were produced while boiling R113 at atmospheric pressure. For both surfaces, the mass flux range was 50–250 kg/m²s and the heat flux range was 5–140 kW/m². The results obtained have been compared with standard correlations for tube bundles.The measured heat-transfer coefficients for the pin-fin surface are slightly higher than those for the plate surface. Both are dependent on heat flux and reasonably independent of mass flux and vapour quality. Thus, heat transfer is probably dominated by nucleate boiling and is increased by the pin fins due to the increase in area and heat-transfer coefficient. The pin-fin pressure drops were typically 7 times larger than the plate values.The pin-fin heat-transfer coefficients and pressure drops are compared to macro-scale tube bundle correlations. At low vapour qualities the heat-transfer coefficients are in reasonable agreement with the correlations, but, as the vapour quality increases, they do not show the convective enhancement which would be expected for a conventionally-sized tube bundle. Measured two–phase pressure drops are in reasonable agreement with the tube bundle correlation.     

Numerical assessment of heat transfer coefficient for preterm infant nursed under a radiant warmer

Body heat loss management is the primary concern with respect to a newborn, as excess heat loss or gain leads to hypo- or hyperthermia, respectively. The aim of this article is to numerically investigate the convection and radiation heat transfer coefficient of a newborn nursed under a radiant warmer. The preterm neonate manikin has five body segments (head, trunk, leg, arms, and back) placed in a relaxed spine position. In the present study, numerical simulations are carried out for body temperature ranging between 32.5°C and 40.1°C. Ambient air temperature is chosen from 22.77°C to 30°C as preferred room temperature in the analysis. Airflow and heat transfer under the radiant warmer are analyzed in two operational modes, that is, power-off and power-on modes. In the power-on mode, the convective heat transfer coefficient varies between 1.45 and 4.06 W/m² K, whereas the radiative heat transfer coefficient varies between 0.08 and 3.28 W/m² K under various operating conditions. The results obtained are numerically validated and found to be in fair agreement with the experimental results available in the open literature.     

Air side heat transfer coefficients of discrete plate finned-tube heat exchangers with large fin pitch

The objective of this study is to investigate the heat transfer characteristics of discrete plate finned-tube heat exchangers with large fin pitches. Thirty-four heat exchangers were tested with variations of fin pitches, the number of tube rows, fin alignment, and vertical fin space. The j-factor of the discrete plate finned-tube exchanger was analyzed as a function of coil geometry and then compared with that of the continuous plate finned-tube heat exchanger. For fin pitches of 7.5–15 mm, the j-factors of the discrete plate finned-tube heat exchangers were 6.0–11.6% higher than those of the continuous plate finned-tube heat exchangers. Two separate correlations for the j-factor were developed for the inline and the staggered fin alignment in the discrete plate finned-tube heat exchangers to predict the measured data within a relative deviation of 2.9%.     

Experimental studies on the behaviours of hydride heat storage system

Experimental studies on the heat transfer characteristics of the heat storage vessel are conducted. Mg₂Ni is used as the heat storage medium. The structure of the heat storage vessel is a single tube in which a number of thermocouples are installed for temperature measurement. The temperature gradient along the axial direction of the tube was not found at the steady state for both heat generation and absorption period. Temperature gradient, however, exists along the radial direction. The overall heat transfer coefficient obtained is about 10 Kcal/m²h deg which implies that heat transfer through the powder bed is a controlling step. The lumped parameter model to describe the performance of the heat storage vessel was derived and it could explain the heat transfer characteristics of the vessel qualitatively.