Water Side Heat Transfer Coefficient Research Articles

Numerical Modelling of Rapid Decrease in Water Flow Rate in a Plate-and-Tube Heat Exchanger

In this article, a mathematical simulation of a plate-fin and tube heat exchanger (PFTHE) is presented. Water flows in the tubes and air flows perpendicular to the tube axis. The general numerical model was developed to perform the transient operation of the heat exchanger. The Reynolds number on the waterside ranged from 4,000 to 12,000. The transient response of the PFTHE was modeled for a rapid decrease in the water volume flow rate in time. The heat transfer correlation for the air side was established using the empirical data. Unknown parameters present in the power-type relationship for the airside Nusselt number were estimated using the least-squares method. The waterside heat transfer coefficients in the transitional and turbulent flow ranges were calculated using the semi-empirical heat transfer correlation. The results of the mathematical simulations of the investigated PFTHE were compared with the measurement data. The agreement between computation and measurement results, i.e., between the calculated and measured air and water temperatures at the outlets of the PFTHE, was very satisfactory.

Analysis and optimization of thermal characteristics in a rotating packed bed

The use of rotating packed bed, as a consequence of process intensification, has been well established in replacement of conventional distillation towers relating to chemical and processing industries, owing to the higher mass transfer rates attainable in the former. The idea of an analogy between heat and mass transfer forged the basis for analyzing the thermal characteristics of the rotating packed bed in this communication. Towards this purpose, 3-D computational fluid dynamics simulation for the data, pertaining to the counter-current multi-phase flow in a rotating packed bed using air-water system, became a persuading task. The entire gamut of information has been synthesized and simplified for interpretation using the central composite rotatable design. The results obtained from the analysis help in assimilation of significant effects imparted by the control parameters on to the heat transfer rate and water side heat transfer coefficient. The use of response surface methodology in the development of surface contours has also been presented.

Experimental Study on the Heat-Transfer Characteristics of a 600 MW Supercritical Circulating Fluidized Bed Boiler

Heat-transfer characteristics such as the distribution of the local heat-flux and the water-side and fire-side heat-transfer coefficients of the water-wall at the dilute phase region of a 600 MW supercritical circulating fluidized bed (CFB) boiler operating at boiler loads of 60%, 80%, and 100%, were experimentally investigated. The temperature of the water-wall at furnace heights of 9 to 55 m was in situ measured using K-type thermocouples, and the water-side heat-transfer coefficient, the distribution of local heat-flux, and the fire-side heat-transfer coefficient between the furnace and water-wall were calculated based on the measured temperature result. The results showed the water-side heat-transfer coefficient was raised from 8600 W·m–2·K–1 to 31000 W·m–2·K–1 as the furnace height increased. Moreover, the water-side heat-transfer coefficient was large enough to ensure the heat-transfer deterioration would not take place in the water-wall tubes. Meanwhile, it was revealed that the heat-flux was highe...

Study of multi-twisted-tube gas cooler for CO2 heat pump water heaters

Multi-twisted-tube type heat exchangers applied in a transcritical CO2 heat pump as gas coolers were investigated. The influence of different numbers of inner tubes on the heat transfer and the pressure drop was discussed by means of experiments and theoretical analysis. The performances of two gas coolers with three and four inner tubes were measured and compared. It is found that heat transfer characteristics of the gas cooler with four inner tubes is better than that with three inner tubes. Based on the theoretical analysis, outlet temperature of water increases with increasing the number of inner tubes at the beginning and the increasing tendency becomes gentle when the number of inner tubes is more than four. But the pressure drop of gas cooler rises greatly with the increment of the number of inner tubes. It is implied from the analysis that the coefficient of performance of CO2 heat pump is not optimum when the outlet temperature of water becomes high by increasing mass flow rate of CO2 and decreasing the mass flow rate of water. In order to get both high COP and high outlet temperature of water, the water side heat transfer coefficient of the gas cooler needs improving.

Development of experimental techniques for measurement of heat transfer rates in heat exchangers in oscillatory flows

Heat exchangers are important components of thermoacoustic devices. In oscillatory flow conditions, the flow and temperature fields around the heat exchangers can be quite complex, and may significantly affect heat transfer behaviour. As a result, one cannot directly apply the heat transfer correlations for steady flows to the design of heat exchangers for oscillatory flows. The fundamental knowledge of heat transfer in oscillatory flows, however, is still not well-established. The aim of the current work is to develop experimental apparatus and measurement techniques for the study of heat transfer in oscillatory flows. The heat transferred between two heat exchangers forming a couple was measured over a range of testing conditions. Three couples of finned-tube heat exchangers with different fin spacing were selected for the experiment. The main parameters considered were fin spacing, fin length, thermal penetration depth and gas displacement amplitude. Their effects on the heat exchanger performance were studied. The results were summarised and analysed in terms of heat transfer rate and dimensionless heat transfer coefficient: Colburn-j factor. In order to obtain the gas side heat transfer coefficient in oscillatory flows, the water side heat transfer coefficient is required. Thus, an experimental apparatus for unidirectional steady test was also developed and a calculation method to evaluate the heat transfer coefficient was demonstrated. The uncertainties associated with the measurement of heat transfer rate were also considered.

Experimental investigation of novel heat exchanger for low temperature lift heat pump

In this paper, the thermal and hydraulic performance of a novel low temperature lift heat exchanger (LTLHX) was experimentally investigated. The novel LTLHX was developed to improve the performance of the conventional plate type heat exchanger (PHX) under low temperature lift operating conditions. The optimized novel LTLHX was fabricated and investigated experimentally with various operating conditions. The heat transfer coefficient correlation and friction factor correlation of the water-side in the novel LTLHX were formulated from the experimental data. The overall heat transfer coefficient of the novel LTLHX with ammonia ranged from 1300 to 2000 W K−1 m−2, and the pressure drop per length of the water-side ranged from 4 to 10 kPa m−1. The refrigerant-side heat transfer coefficient ranged from 2900 to 5000 W K−1 m−2, and water-side heat transfer coefficient ranged from 3900 to 5100 W K−1 m−2. The overall heat transfer performance of the novel LTLHX was more than double that of the PHX at the same operating conditions. Moreover, the water-side pressure drop of the novel heat exchanger was drastically lower than that of the PHX. It was due to the balanced thermal and hydraulic performance of the novel heat exchanger.

Thermal and hydraulic performance of sinusoidal corrugated plate heat exchanger for low temperature lift heat pump

Thermal and hydraulic performance of a sinusoidal corrugated plate heat exchanger (PHX) was investigated for the application of a low temperature lift heat pump (LTLHP), which requires unique operating conditions. The water-side heat transfer coefficient and pressure drop of the PHX were obtained through the experimental test. The refrigerant-side heat transfer performance was investigated by varying several parameters. The PHX performance was poor due to low refrigerant mass flux. The PHX needs better balance in two fluids for the LTLHP application. From the current study, it is concluded that the conventional PHX applied for the LTLHP application is limited by two main factors: a large pressure drop on the water-side due to corrugated shape, and a low heat transfer performance due to the low refrigerant-side heat transfer performance. In order to address these drawbacks, heat exchanger designs must be improved by optimizing its geometry and flow area asymmetrically for each fluid.

Measurements of absorbed heat flux and water-side heat transfer coefficient in water wall tubes

Measurements of absorbed heat flux and water-side heat transfer coefficient in water wall tubes The tubular type instrument (flux tube) was developed to identify boundary conditions in water wall tubes of steam boilers. The meter is constructed from a short length of eccentric tube containing four thermocouples on the fire side below the inner and outer surfaces of the tube. The fifth thermocouple is located at the rear of the tube on the casing side of the water-wall tube. The boundary conditions on the outer and inner surfaces of the water flux-tube are determined based on temperature measurements at the interior locations. Four K-type sheathed thermocouples of 1 mm in diameter, are inserted into holes, which are parallel to the tube axis. The non-linear least squares problem is solved numerically using the Levenberg-Marquardt method. The heat transfer conditions in adjacent boiler tubes have no impact on the temperature distribution in the flux tubes.

Numerical Analyses of Effects of Tube Shape on Performance of a Finned Tube Heat Exchanger

Th is article discusses the eff ects of tube geometry on the performance of a multi-row fi nned tube heat exchanger having herringbone wavy fi ns. Th e air-side heat transfer and pressure drop characteristics of the heat exchanger were predicted numerically and the tube-side heat transfer coeffi cient and pressure drop were calculated using commonly adopted equations for turbulent fl ow in a smooth tube. Th e numerical calculations were performed in three dimensions for three frontal air velocities of 1.0, 2.0, and 3.0 m/s, yielding hydraulic diameter Reynolds numbers from 297 to 999. Investigated were fi ve tube geometries, including a round tube, three elliptical oval tubes, and a fl at oval tube. Th e round tube has a 15.875 mm (⅝ ) outside diameter and serves as the baseline tube. Th e four oval tubes were made by reforming the baseline round tube and have the same perimeter as that of the round tube. Th e aspect ratios of the three elliptical tubes are 2.00, 3.00, and 4.29, respectively, and the aspect ratio of the fl at oval tube is 3.00. As the tube aspect ratio is increased, the air-side heat transfer coeffi cient and pressure drop decrease, but the water-side heat transfer coeffi cient and pressure drop increase. At 2.0 m/s frontal velocity, the 3.00 aspect ratio elliptical tube yields a 6.9% lower air-side heat transfer coeffi cient and a 45.9% lower air pressure drop than the round tube. As compared to the 3.00 aspect ratio elliptical tube, the same aspect ratio fl at tube has a 0.6% higher air-side heat transfer coeffi cient and a 2.5% higher air pressure drop.

Prediction of two-phase heat transfer in a 4-pass evaporator bundle using single tube experimental data

Two-phase heat transfer and pressure drop on the tube-side of a 4-pass R-22 microfin tube bundle is modeled using experimental data on a single tube. The modeled direct expansion bundle has water flow on the shell side. The modeled bundle is divided into small axial incremental lengths along the refrigerant path. Heat transfer and pressure drop calculations are done simultaneously. The evaporation heat transfer coefficients and pressure drop are calculated using the single tube data taken at flow conditions characterizing the tube bundle. The water side heat transfer coefficient was determined by a Wilson Plot calibration for the shell side of the tube bundle. The bundle predictions are compared with the experimental data on the modeled tube bundle. The model predicted the heat transfer for two tube bundles having different microfin tubes with less than 6% error. The simulations showed that ±5% variation from the assumed 85% dry-out vapor quality the bundle heat load varied ±3.5%. Flow mal-distribution analysis showed that moderate flow mal-distribution does not cause significant performance loss. A parametric analysis was performed to define the optimum number of tubes in each pass.

Simplified model for indirect-contact evaporative cooling-tower behaviour

A simplified model for indirect cooling towers behaviour is presented. The model is devoted to building simulation tools and fulfils several criteria such as simplicity of parameterisation, accuracy, possibility to model the equipment under various operation conditions and short computation time. On the basis of Merkel's theory, the model is described by using the Effectiveness-NTU method. The model introduces only two parameters, air-side and water-side heat-transfer coefficients which can be identified from only two rating points, data easily available in manufacturers' catalogues. Thus, the model allows one to estimate energy and water consumptions under different operating conditions such as variable wet-bulb temperatures or variable airflow rates.

Fouling in enhanced tubes using cooling tower water: Part I: long-term fouling data

This paper describes the results of long-term fouling tests for cooling tower water flowing inside enhanced tubes. The data were taken using 800 ppm calcium hardness water supplied to an operating chiller/cooling tower system. The fouling mechanism is a combination of precipitation fouling and particulate fouling. Fouling data were measured for seven different enhanced tube geometries over a 2500 h operating period. The tubes tested had internal helical ridges, and the number of ridge starts varied from 10 to 45. Little fouling occurred until 1300 h. After 1300 h, moderate fouling began to occur in the tubes having a large number of starts. These tubes also provide the highest water-side heat transfer coefficients. Significant fouling existed in the tubes having 30–45 starts at the end of the test period. The salient finding of this work is that the potential for fouling increases as the number of starts and helix angle increases.

ディーゼル船排ガスエコノマイザのスート燃焼

The so-called“soot fire”, whichi is a heating surface melt-down failure of the exhaust gas economizer (EGE) of diesel ship, is so serious that it has been urgently waiting for effective countermeasures. This report developes a simulation model in order to investigate the sootburning and heat transfer process in the EGE, of which heating surface consists of finned tubes with parallel arrangement. The heat transfer equation of the model takes into consideration 4 factors, that is, the heat transported by the gas flow, the heat transmitted from gasside to waterside, the heat being accumulated in the finned tube materials, and the heat released by the soot burning. The soot burning was modelled by a surface combustion of a soot layer deposited on heating surface. The simulation model can explain successfully various empilical facts reported so far regarding actual ship's soot fire failures. The calculation results give valuable informations regarding the influences of gasside and waterside heat transfer coefficients, gas flow rate, soot carolific value, soot deposition thickness etc. Among them, the waterside heat transfer coefficient, that is, the condition of water circulation shows the most definite influence on the soot burning, and it is made clear that the soot burning cannot occur the melt-down failure when the water circulation is sufficient to prevent the dryout.

Performance test of a shell-and-plate-type condenser for OTEC

The performance test of a shell-and-plate-type condenser for ocean thermal energy conversion (OTEC) plants is reported. The plate has the following dimensions: ratio of rib height to the equivalent diameter, k s / D eq = 2.25; length, 1450 mm; width, 235 mm; thickness, 1.0 mm. The number of plates used in the condenser is 168; the length of the actual heat transfer area of the plate of 0.9 m, and the actual total surface area is 40.7 m 2. Freon 22 is used as working fluid. The overall heat transfer coefficient is about 2500 W/(m 2 K) when the cooling water velocity is 1 m/s and the vapor inlet temperature is in the range of 22.8–23.2°C. Empirical correlations are proposed for predicting the average condensation heat transfer coefficient and the water-side heat transfer coefficient. The water-side pressure drop is 1.7 × 10 4 N/m 2 when the cooling water velocity is 1 m/s. The water-side friction factor is about 0.25 at a Reynolds number of 4 × 10 3.

Film Condensation of Steam on Horizontal Finned Tubes: Effect of Fin Spacing

The film condensation heat transfer performance of six externally finned copper tubes has been evaluated. All tubes had rectangular-shaped fins with a height and thickness of 1 mm. The spacing between fins was 0.5, 1.0, 1.5, 2.0, 4.0, and 9.0 mm. Data were also obtained for a smooth tube whose outside diameter of 19.0 mm was equal to the diameter at the base of the fins for all of the finned tubes. Tests were performed both at atmospheric pressure and under vacuum (∼ 11.3 kPa). Steam flowed vertically downward with a velocity of approximately 1 and 2 m/s at atmospheric pressure and under vacuum, respectively. The smooth tube was fitted with wall thermocouples for the evaluation of the water-side heat transfer coefficient. This was used, subsequently, to determine the steam-side heat transfer coefficient for the finned tubes for which only overall measurements were made. Strenuous efforts were made to obtain high-accuracy data; in particular, the coolant temperature rise was determined by both quartz-crystal thermometers and a 10-junction thermopile. The two temperature-rise measurements always agreed to within ± 0.03 K. Care was taken to avoid errors due to the presence of noncondensing gases and to ensure that filmwise condensation conditions prevailed over the entire tube throughout all tests. The steam-side heat transfer coefficient for the smooth tube agreed closely with values found by other recent workers. Maximum steam-side enhancement was found for the tube with a fin spacing of 1.5 mm. At this fin spacing, the heat transfer enhancement ratios were around 3.6 and 5.2 for low-pressure and atmospheric pressure runs, respectively.

Heat and mass transfer from a single horizontal tube of an evaporative tubular heat exchanger

In this investigation, water side heat transfer coefficients without air flow from a single horizontal tubes are determined. Mass transfer coefficients are determined with water and air flow from the same tube. The total energy dissipated by inside hot fluid when only water is falling is compared with that when both the air and water flow past the tube. The water side heat transfer coefficient and mass transfer coefficient are given by empirical relations h w = 6.0(Re p) 0.18(Re w) 0.87 and K = 3.5(Re p) 0.18(Re a) 0.28 (Re w) 0.54, respectively. The ratio of energies dissipated with water and air flow and with only water flow increases with Re w and Re a and its maximum value is 1.72 in the range of variables used.