Experimental Study on the Heat Transfer Characteristics of an Evaporating Falling Film on a Horizontal Plain Tube

In this paper, an experimental investigation on the heat transfer of saturated water falling film on a single horizontal plain tube is presented. The water film falling on the outside of the tube has been heated by the condensing steam flowing in the tube, and the heat transfer coefficient between the water film and the steam has been measured. Experiments were performed at saturation temperatures of liquid film and steam as 58°C and 61°C, and 61°C and 65°C, a tube pitch of 57.16 mm, heat fluxes from 10 to 50 kW m^-2, and film flow rate per unit of length of the tube up to 0.12 kg m^?1 s^?1. Brass plain tubes with external diameters of 25.4 mm and lengths of 950 mm were used in the experiments. The experimental results show that the heat transfer coefficient increases with the increasing film flow rate and heat flux, and the quality of vapor has an obvious influence on the heat transfer performance of falling film evaporation. The coupling of condensation and evaporation heat transfer inside and outside the tube is investigated qualitatively in this paper.     

Experimental investigation of capillary-assisted evaporation on the outside surface of horizontal tubes

Capillary-assisted evaporation is a typical heat transfer method in heat pipes which is characterized by high evaporation coefficient due to extremely thin liquid film. This paper introduces such a micro-scale heat transfer method into normal-scale applications. A series of enhanced heat transfer tubes with circumferential rectangular micro-grooves on the outside surfaces have been experimentally investigated. The aim is to investigate the influence of the tubes’ geometries and operating parameters on the evaporation heat transfer coefficients. In the experiment, the tested tubes are hold horizontally and the bottom surfaces are immersed into a pool of liquid. The heat is added to the thin liquid film inside the micro-grooves through the heating fluid flowing inside the tubes. The factors influencing the capillary-assisted evaporation performance, such as the immersion depth, evaporation pressure, superheating degree, etc. are considered. The experimental results have indicated that there is a positive correlation between the evaporation heat transfer coefficient and evaporation pressure, and negative for the superheating and immersion depth. For water, under the evaporation saturated temperature of 5.0 ± 0.1 °C, the superheating of 4.0 ± 0.1 °C and the dimensionless liquid level of 1/2, the film side evaporation heat transfer coefficients are 3100–3500 W/m² K, which are equivalent to those of the falling film evaporator in LiBr–water absorption machine (2800–4500 W/m² K [Y.Q. Dai, Y.Q. Zheng, LiBr–water Absorption Machine, first ed., Chinese National Defence Industry Press, Beijing, China, 1980.]).     

Falling water film evaporation on newly-designed enhanced tube bundles

Experimental studies are presented on falling film evaporation of water on 6-row horizontal enhanced tube bundles in a vacuum condition. Turbo-CAB (19 fpi and 26 fpi), Korodense, and smooth tubes were tested in a range of film Reynolds number from about 10 to 110 and in the condition of only convective evaporation, without nucleate boiling. The flow modes and heat flux will affect the transition Reynolds numbers. Tubes with enhanced inner surface provide better heat transfer performance. Hotter heating water may lead to better heat transfer performance mainly due to higher heat fluxes. Correlations were also derived to predict the heat transfer coefficients and the enhancement ratio.     

Falling Liquid Film Evaporation in Subcooled and Saturated Water Over Horizontal Heated Tubes

ABSTRACT

This paper presents an experimental study of heat transfer and film thickness behavior in falling liquid water film evaporation technology over horizontal tubes. Liquid distribution systems have also been evaluated. The experimental setup consisted of two horizontal 0.019-m OD stainless-steel tubes, 0.194 m in length. Reynolds numbers in the 160–940 range were tested in both subcooled and saturated liquid regimes. For the liquid distribution system study, several distributor geometries were tested in order to develop the least disturbed film over the tubes. An intrusive method was used for measuring the liquid film thickness in the laminar regime and the measured values were compared with the theoretical prediction computed from the Nusselt equation. An experimental heat transfer correlation was obtained and compared with previous ones obtained by other authors. In addition, the local heat transfer coefficient was observed to be always higher at the horizontal tube top region for all operational conditions (on the order of 14 kW/m²-°C). Finally, the use of a liquid storage distribution system along with the installation of a wire mesh to obtain an uniform liquid distribution for all Reynolds numbers tested.     

低温地热双循环式发电系统横管喷淋降膜蒸发器传热性能实验研究

以R245fa为工质,搭建有机朗肯循环（ORC）发电系统横管喷淋降膜蒸发器传热测试平台,研究有机工质喷淋密度,地热水初温及流率等因素对管外换热系数的影响。实验结果表明：随着有机工质喷淋密度、地热水初温、地热水流率的增大,传热系数均先增大后减小。最后,根据实验结果,对现有横管喷淋降膜蒸发器的管外传热系数经验公式的参数进行修正。     

Heat transfer characteristics of falling film evaporation on horizontal tube arrays

A falling film heat transfer test facility has been built for the measurement of falling film evaporation in a vacuum of about 1000 Pa. At this condition, only convective evaporation occurred in the liquid film. The Reynolds numbers of falling film over a range from 21.6 to 108.1 were tested on six-tube arrays made of enhanced or smooth tubes. Results show that the tubes with both enhanced outer and inner surfaces give high heat flux. Besides, as the Reynolds number increases, the heat transfer enhancement ratio of falling film evaporation decreases. A semi-analytical correlation is established to predict the heat transfer coefficients of falling film evaporation on smooth tube arrays, considering the contributions of partially dryout and fully wet regimes, respectively. For enhanced tubes, the heat transfer enhancement ratios to the smooth tubes were also correlated.     

Heat and mass transfer of ammonia-water in falling film evaporator

To investigate the performance of heat and mass transfer of ammonia-water during the process of falling film evaporation in vertical tube evaporator, a mathematical model of evaporation process was presented, the solution of which that needed a coordinate transformation was based on stream function. The computational results from the mathematical model were validated with experimental data. Subsequently, a series of parameters, such as velocity, film thickness and concentration, etc., were obtained from the mathematical model. Calculated results show that the average velocity and the film thickness change dramatically at the entrance region when x<100 mm, while they vary slightly with the tube length in the fully developed region when x>100 mm. The average concentration of the solution reduces along the tube length because of evaporation, but the reducing tendency becomes slow. It can be concluded that there is an optimalβrelationship between the tube length and the electricity generated. The reason for the bigger concentration gradient in the y direction is that the smooth tube is chosen in the calculation. It is suggested that the roll-worked enhanced tube or other enhanced tube can reduce the concentration gradient in the film thickness direction and enhance the heat and mass transfer rate.     

Evaporation heat transfer of falling water film on a horizontal tube bundle

A numerical simulation and experimental study were carried out for evaporation heat transfer of a falling water film on a smooth horizontal tube bundle evaporator. A laminar model and a turbulence model were respectively adopted to calculate the heat transfer coefficients of falling water film on horizontal heated tubes. The calculation zone on the heated tube was divided into the top stagnation zone and the lateral free film zone. The initial boundary conditions for the free film zone were determined from the calculated results of the stagnation zone. The modified wall function method was used for the turbulent flow. Comparisons between the experimental data and the numerical solutions by use of two flow models show that the experimental data lie between the laminar model solutions and the latter turbulence model solutions and that they are closer to the latter solutions. Finally, a simple dimensionless correction based on the numerical simulations is proposed for predicting the evaporation heat transfer of falling water film for actual engineering applications. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 42–55, 2002     

Falling film evaporation characteristics of microalgae suspension for biofuel production

Microalgae, one of the important biofuel producers, have received considerable attention recently. Dewatering is one of the bottlenecks for its industrialization due to the dilute nature of the suspensions and the small cell size. Traditional liquid–solid separation processes are not efficient for dewatering of microalgae suspensions. In this study, falling film evaporation was employed for dewatering of microalgae suspension, which is a popular process for concentrating heat sensitive materials. The heat transfer coefficient was as high as 9414.20 W/m² K with mass flow rate of 0.233 kg/s, ΔT of 1.21 °C, and microalgae concentration of 60 g/L. The falling film evaporation process can be made highly energy efficient if it is coupled with Mechanical Vapor Recompression (MVR) or Thermal Vapor Recompression (TVR) system. Heat and mass transfer characteristics of falling film evaporation of microalgae suspension have been investigated here. This will provide the fundamentals for future feasibility study of utilizing the falling film evaporation in the microalgal industry.     

Absorption of water vapour in the falling film of water–lithium bromide inside a vertical tube at air-cooling thermal conditions

In the absorbers of air-cooled water–lithium bromide absorption chillers, the absorption process usually takes place inside vertical tubes with external fins. In this paper we have carried out an experimental study of the absorption of water vapour over a wavy laminar falling film of water–lithium bromide on the inner wall of a smooth vertical tube. The control variables for the experimental study were; absorber pressure, solution mass flow rate, solution concentration and cooling water temperature. Relatively high cooling water temperatures were selected to simulate air-cooling thermal conditions. The parameters considered to assess the performance of the absorber were; the mass absorption flux, the outlet solution degree of subcooling and the falling film heat transfer coefficient. The results indicate that in water cooling thermal conditions the mass absorption fluxes are in the range 0.001–0.0015 kg·m⁻²·s⁻¹, whereas in air-cooling thermal conditions the range of mass absorption values decreases to 0.00030–0.00075 kg·m⁻²·s⁻¹.     

Experimental study on condensation and evaporation flow inside horizontal three dimensional enhanced tubes

Experimental investigations of tube side condensation and evaporation in two 3-D enhanced heat transfer (2EHT) tubes were compared to the performance of a smooth surface copper tube. The equivalent outer diameter of all the tubes was 12.7 mm with an inner diameter of 11.5 mm. Both the inner and outer surfaces of the 2EHT tubes are enhanced by longitudinal grooves with a background pattern made up by an array of dimples/embossments. Experimental runs were performed using R410A as the working fluid, over the quality range of 0.2–0.9. For evaporation, the heat transfer coefficient ratio (compares the heat transfer coefficient of the enhanced tube to that of a smooth tube) of the 2EHT tubes is 1.11–1.43 (with an enhanced surface area ratio of 1.03) for mass flux rate that ranges from 80 to 200 kg/m² s. For condensation, the heat transfer coefficient ratio range is 1.1–1.16 (with an enhanced surface area ratio of 1.03) for mass flux that ranges from 80 to 260 kg/m² s. Frictional pressure drop values for the 2EHT tubes are very similar to each other. Heat transfer enhancement in the 2EHT tubes is mainly due to the dimples and grooves in the inner surface that create an increased surface area and interfacial turbulence; producing higher heat flux from wall to working fluid, flow separation, and secondary flows. A comparison was performed to evaluate the enhancement effect of the 2EHT tubes using a defined performance factor and this indicates that the 2EHT tubes provides a better heat transfer coefficient under evaporation conditions.     

New proposed two-phase multiplier and evaporation heat transfer coefficient correlations for R134a flowing at low mass flux in a multiport minichannel

New correlations of the two-phase multiplier and heat transfer coefficient of R134a during evaporation in a multiport minichannel at low mass flux are proposed. The experimental results were obtained from a test using a counter-flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Test section is composed of the extruded multiport aluminium inner tube with an internal hydraulic diameter of 1.2 mm and an acrylic outer tube with an internal hydraulic diameter of 25.4 mm. The experiments were performed at heat fluxes between 10 and 35 kW/m², and a refrigerant mass flux between 45 and 155 kg/(m² s). Some physical parameters that influenced the frictional pressure drop and heat transfer coefficient are examined and discussed in detail. The pressure drop and heat transfer coefficient results are also compared with existing correlations. Finally, new correlations for predicting the frictional pressure drop and heat transfer coefficient at low mass fluxes are proposed.     

Cross Vapor Stream Effect on Falling Film Evaporation in Horizontal Tube Bundle Using R134a

The falling film evaporation of R134a with nucleate boiling outside a triangular-pitch (2-3-2-3) tube bundle is experimentally investigated, and the effects of saturation temperature, film flow rate and heat flux on heat transfer performance are studied. To study the effect of cross vapor stream on the falling film evaporation, a novel test section is designed, including the tube bundle, liquid and extra vapor distributors. The measurements without extra vapor are conducted at the saturation temperature of 6, 10 and 16°C, film Reynolds number of 220 to 2650, and heat flux of 20 to 60 kWm^?2. Cross vapor stream effect experiments are operated at three heat fluxes 20, 30, and 40 kWm^?2 and two film flow rates of 0.035 and 0.07 kgm^?1s^?1, and the vapor velocity at the smallest clearance in the tube bundle varies from 0 to 2.4 ms^?1. The results indicate that: film flow rate, heat flux and saturation temperature significantly influence the heat transfer; the cross vapor stream either promote or inhibit the falling film evaporation, depending on the tube position, film flow rate, heat flux and vapor velocity.     

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.     

Experimental study of evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal small tubes

An experiment is carried out here to investigate the characteristics of the evaporation heat transfer for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m² s, imposed heat flux q from 5 to 15 kW/m², inlet vapor quality x_in from 0.2 to 0.8 and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the 0.83-mm tubes, G is varied from 800 to 1500 kg/m² s with the other parameters varied in the same ranges as those for D_i = 2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. The experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rises in the imposed heat flux, refrigerant mass flux and saturation temperature. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the smaller tubes (D_i = 0.83 mm) may decline at increasing vapor quality when the quality is high, due to the partial dryout of the refrigerant flow in the smaller tubes at these conditions. We also note that under the same x_in, T_sat, G, q and D_i, refrigerant R-407C has a higher h_r when compared with that for R-134a. Finally, an empirical correlation for the R-134a and R-407C evaporation heat transfer coefficients in the small tubes is proposed.     

Correlation of Evaporation Heat Transfer Inside 8.8 mm and 7.14 mm Horizontal Round Micro-Fin Tubes

Experimental investigations of evaporation heat transfer of R22 and R410A inside a horizontal micro-fin copper tube have been conducted and are reported here. Six micro-fin tubes with inner diameter of 7.14 mm and three micro-fin tubes with inner diameter of 8.8 mm but with different geometric parameters, such as the apex angle, the helical angle, fin height, fin pitch, and starts were tested. The evaporation experiments were taken at a constant temperature of 6°C. Moreover, working conditions of the experiments varied with the mass flux ranging from 100 kg/(m².s) to 400 kg/(m².s). For the evaporation experiments of Tube 1 – Tube 6 with R22, the inlet and outlet vapor quality is set as 0.1 and 0.9, respectively. For the evaporation experiments of Tube 7 – Tube 9 with R410A, the inlet and outlet vapor quality is set as 0.2 and 0.9, respectively. The heat transfer coefficients and the changing trend of the heat transfer coefficients vary among these tubes. The influence of each geometric parameter on the heat transfer performance of the micro-fin tube has been analyzed and is reported. Besides, correlations of evaporation heat transfer inside 8.8 mm or less horizontal round micro-fin tubes were developed.     

Mathematical simulation of lithium bromide solution laminar falling film evaporation in vertical tube

For utilization of the residual heat of flue gas to drive the absorption chillers,a lithium-bromide
falling film in vertical tube type generator is presented.A mathematical model was developed to
simulate the heat and mass coupled problem of laminar falling film evaporation in vertical tube.In the
model,the factor of mass transfer was taken into account in heat transfer performance calculation.The
temperature and concentration fields were calculated.Some tests were conducted for the factors
such as Re number,heating flux,the inlet concentration and operating pressure which can affect the
heat and mass transfer performance in laminar falling film evaporation.The heat transfer performance is
enhanced with the increasing of heat flux.An increasing inlet concentration can weaken the heat
transfer performance.The operating pressure hardly affects on heat and mass transfer.The bigger inlet
Re number means weaker heat transfer effects and stronger mass transfer.The mass transfer
obviously restrains the heat transfer in the falling film solution.The relation between dimensionless
heat transfer coefficient and the inlet Re number is obtained.     

海水淡化中降膜蒸发过程的实验研究

由于横管降膜蒸发具有传热系数高、热耗低、可利用低品位能源等优点,横管降膜蒸发技术成为低温多效海水淡化技术中应用最广泛的方法.搭建了单管横管降膜试验台,观测了横管降膜蒸发的流动过程,研究了横管降膜蒸发过程中的流动特性和传热特性,归纳了管间距、喷淋密度对横管降膜蒸发过程中流动特性和传热特性的影响.实验结果表明:喷淋密度的增...     

Analysis of effective wetting area of a horizontal generator for an absorption heat transformer

The use of thermal processes for water purification is a good alternative to reduce primary energy consumption. The efficiency in heat exchange process by falling film plays a very important role in the performance of a heat pump. The objective of this paper is to evaluate the efficiency of heat transfer on a tube bank, of a heat transformer designed for water purification with LiBr–H₂O as absorbent mixture, by digital image processing. To analyze the wetting area and falling film behavior of the mixture on a bank of sixteen horizontal tubes, of 1.22 cm outer diameter and 30 cm in length, experimental tests were performed. The results show the distribution of absorbent mixture, along the tubes of the bank, with 0 and 5° inclination respect to the horizontal, and different mass flow rates per unit length (0.006–0.034 kg/m s). The film thickness of LiBr–H₂O was determined by the digital image processing, obtaining an average value of 0.033 cm. The most regular fall of mixture, through the tubes of the bank, was obtained with the flow of 0.025 kg/m s. This mass flow was the one which gave the best results of efficiency of heat transfer, with values between 80 and 98%.     

Evaporation Heat Transfer Coefficients of R-446A

ABSTRACT

The present study investigated the evaporation heat transfer coefficients of R-446A, as a low global warming potential alternative refrigerant to R-410A. The evaporation heat transfer coefficients were obtained by measuring the wall temperature of a straight stainless tube and refrigerant pressure. The heat transfer coefficients were measured for the quality range from 0.05 to 0.95, the mass flux from 100 to 400 kg/m²s, heat flux from 10 to 30 kW/m², and saturation temperature from 5 to 10°C. The evaporation heat transfer coefficient of R-410A was verified by comparing the measured evaporation heat transfer coefficient with the value predicted by the existing correlation. The evaporation heat transfer coefficient of R-446A was measured using a proven experimental apparatus. When the heat flux was 10 kW/m², the evaporation heat transfer coefficient of R-446A was always higher than that of R-410A. But, when the heat flux was 30 kW/m², the evaporation heat transfer coefficient of R-446A was measured to be lower than that of R-410A near the dry-out point. The effect of the tube diameter on the R-446A evaporation heat transfer coefficient was negligible. The effect of saturation pressure on the evaporation heat transfer coefficient was prominent in the low quality region where the nucleate boiling was dominant.