Falling Film Evaporation Heat Transfer Research Articles

Experimental study on falling film evaporation heat transfer outside a horizontal tube in refrigeration system

Experimental study on falling film evaporation heat transfer outside a horizontal tube in refrigeration system

On falling film evaporator – A review of mechanisms and critical assessment of correlation on a horizontal tube bundle with updated development

Optimization of the energy efficiency and reduction of the greenhouse gas emission for the heat pump system is imperative to meet the net-zero goals at 2050. Chillers with falling film evaporator design not only possess better system performance but also contain less refrigerant inventory. Hence, accurate prediction of the evaporator performance is pivotal especially when charged with low-GWP refrigerant. The present study reviews the correlations for falling film evaporator with a horizontal tube bundle configuration. The major efforts of this study include four tasks: (a) literature review of the experimental studies and available empirical correlations; (b) comprehensive discussion of the falling film evaporation heat transfer mechanism; (c) development of a new rationally based correlation based on available literature; and (d) comparison of different correlations based on the existing data. The collected data includes 4114 data points from 8 sources, 6 refrigerants (R-600a, R-290, R-245fa, R-134a, R-1234ze(E), R-123), 5 types of the tubes (Plain, Turbo-GII-HP, GEWA-B, Low-fin, High-Flux), liquid Weber number from 2.2 × 10−6 to 0.7, imposed vapor Weber number from 0 to 37.2, heat flux from 2.5 to 151.5 kW/m2, and film Reynolds number from 1 to 3159.8. The new correlation gives a MAD of 28.3%, and an R2 of 0.86. Yet, the developed correlation considers various heat transfer mechanisms, including the transition point from falling film evaporation to the nucleate boiling, local evaporation, dry-out, mist flow, imposed flow, and enhanced tube effects.

Heat transfer coefficient correlations of water subatmospheric vaporization in a channel of a smooth plate heat exchanger, based on Vaschy-Buckingham theorem

Water as an environmentally friendly, safe and cheap natural refrigerant (R-718) requires vaporization conditions at sub-atmospheric pressures (∼1 kPa). At these pressure ranges, studies on the specific vaporization of water are scarce and insufficient. For this reason, the engineering of heat exchangers using R-718 as working fluid remains empirical. The aim of this paper is thus to provide correlations to estimate water pool boiling and falling film evaporation heat transfer coefficient under subatmospheric pressure. Correlations developed are believed to be more widely used by providing dimensionless numbers consistent with phase-change phenomena occurring at sub and at-mospheric pressure. To develop these correlations, heat transfer phenomena during water vaporization at pressures ranging from 0.7 to 1.7 kPa in a scale-lab size (0.2 m wide × 0.5 m high) vertical smooth plate evaporator channel are studied. Filling ratio vary from 1/2 to 1/10 of the total channel height, canal thicknesses are set to 2 mm, 4 mm or 6 mm. Thus, raw experimental data from 139 tests performed under various operating conditions of a thermosyphon loop mimicking a sorption system are exploited. Each test consists of more than 1200 values. A graphical and statistical comparison between the heat transfer coefficient (HTC) obtained from the experimental database and the HTC estimated by empirical correlations from the literature is made, calculating the percentage of predicted data within an error band of ±30% (τ30), the mean absolute error (MAE), the mean relative error (MRE) and the correlation coefficient (r). Two methodical correlations for predicting the HTC of pool boiling and liquid film evaporation, are developed. They predict 86% (with MAE = 19%, MRE = 3%, r=0.79) and 83% (with MAE = 18%, MRE = 2%, r=0.84) of the experimental data in τ30 respectively. To extend their validity domains, the proposed correlations are evaluated using independent experimental data from the literature. Giving satisfactory results, it constitutes a first step towards the development of tools used for the sizing and the optimization of the design of two-phase heat exchangers.

Experimental study on enhanced heat transfer tubes in falling film evaporation

The vertical tube falling film evaporation experimental platform was built, and the falling film evaporation heat transfer experiment was carried out. Comparative study was carried out between two types of enhanced heat transfer tubes, which are converging-diverging tube and the transversally corrugated tube, to explore their heat transfer characteristics of falling film evaporation. The results show that the falling film evaporative heat transfer coefficient of the two tubes increases with the increase of the unit peripheral flow rate, the heat transfer temperature difference, heat flux and the Reynolds number of the flash steam. The falling film evaporation heat transfer coefficient of the converging-diverging tube is about 1.09 times that of the transversally corrugated tube. According to the experimental data, the falling film evaporation heat transfer correlations of these two types of tubes were obtained. These results provide a useful reference for the using of transversally corrugated tube and converging-diverging tube in vertical falling film evaporator.

Falling film evaporation and pool boiling heat transfer of R1233zd(E) on thermal spray coated tube

• Falling film evaporation heat transfer of R1233zd(E) was experimentally investigated. • Up to 4.2-fold heat transfer enhancement could be obtained by thermal spray coating. • R1233zd(E) achieved a lower heat transfer coefficient than R134a at 20 °C. Falling film evaporators have been expected as an alternative to flooded evaporators for reducing the refrigerant usage. It is important for the enhancement of the film evaporation heat transfer to keep a liquid film and promote nucleate boiling in the liquid film. This study focused on nucleate boiling heat transfer in the liquid film by thermal spray coating that can be applied regardless of the material of heat transfer tubes. The heat transfer performance of falling film evaporation on a horizontal copper tube was experimentally evaluated. Since nucleate boiling phenomena, such as activation of nucleation sites and bubble behaviors in the liquid film, strongly depend on the physical properties of the refrigerant, the experimental results for R1233zd(E), one of the alternative refrigerants with low GWP, were compared with those for R134a. Two types of tubes were used: the smooth tube and the thermal spray coated tube fabricated by an arc wire spraying method, with the outer diameter of 19.05 mm and the heating length of 50 mm. Pool boiling experiments were also conducted to confirm the wall superheat for nucleate boiling. The heat transfer coefficients were evaluated with a film mass flow rate range of 8.5 × 10 −3 to 6.3 × 10 −2 kg/(m·s), a heat flux range of 10 to 85 kW/m 2 , and a saturation temperature at 20 °C. The effects of the structure of the heating surface, heat flux, and thermophysical properties of the refrigerant on the heat transfer performance are discussed. Results show that the heat transfer of the falling film became the nucleate boiling dominant with a sufficient heat flux and liquid film flow rate. The thermal spray coated surface enhanced not only the nucleate boiling but also the liquid spreading by the vapor bubbles in the liquid film, and thus the coated tube produced higher heat transfer coefficients of 2.1 to 4.8 times those for the smooth tube. The wall superheat at the onset of nucleate boiling was higher for R1233zd(E) than for R134a, because R1233zd(E) has a higher surface tension and lower vapor density. Thus, the pool boiling and falling film evaporation heat transfer coefficients were lower for R1233zd(E) than for R134a.

Falling Film Evaporation Heat Transfer of R 134a and R 1233zd(E) on a Horizontal Low Finned Tube with Thermal Spray Coating

Falling Film Evaporation Heat Transfer of R 134a and R 1233zd(E) on a Horizontal Low Finned Tube with Thermal Spray Coating

Experimental investigation on the heat transfer performance of falling film evaporation with lubrication oil on horizontal tubes having different structures

Falling film evaporation which has the advantages of refrigerant charge reduction and heat transfer improvement has been widely used in refrigeration and heat pump systems. Considering the inescapable and complicated influences of lubricating oil, the falling film evaporation heat transfer of R134a with POE-type lubricating oil entrained from 0.5% to 5.1% on horizontal tubes was examined. Experiments were conducted using test tubes with different surface structures at spray density of 0.0065–0.0105 kg/(s·m) and heat fluxes of 12–65 kW/m2. The saturation temperature was set to 6 °C. Experimental results showed that the heat transfer coefficients of the enhanced tubes were higher than that for the smooth tube and the lubricating oil could be conducive to the heat transfer of the tested tubes. The heat transfer coefficient of enhanced tube #1 was increased by more than 30% when the oil content was 5.1%. The mechanism of the function of lubricating oil on the heat transfer is closely related to the surface structure. For tube #1, oil content always supported heat transfer. In the case of tube #2, an optimal oil content at low spray density may exist.

Experimental study of the falling film evaporation coefficients of R290 in a horizontal enhanced tube array

In this study, the falling film evaporation heat transfers of R290 on an array composed of five enhancement horizontal tubes (groove tubes) are studied. The tests are performed at constant saturation temperatures 5.5 °C with change of heat flux from 10 to 40 kW/m2. The film Reynolds number ranges from 200 to 2200 and the film flow rate of refrigerant is between 100 and 660 kg/h.The results show that the film flow rate and heat flux have significant effects on R290 falling film evaporation heat transfer coefficients (HTCs) of the tubes in the tube array. With decreasing the film flow rate on the five tubes the tube HTCs display two stages, a plateau stage and a sharp drop stage. The heat transfer coefficients firstly keep more or less constant at the plateau stage and then decreasing rapidly. At the same nominal film Reynolds number the tube averaged heat transfer coefficients of tube No. 1 to tube No. 5 decrease in order of the increasing tube number from top to bottom of the array. The falling film evaporation HTC of R290 on single enhanced tube is about 4.5 times of single smooth tube HTC, and the HTCs of the enhanced tube array is higher than single smooth tube by more than 2.5 times. In addition, the R290 HTCs of the tube array are higher than those of R134a for the same tube array in the plateau region by about 25%. It is found that at high heat flux of 30–40 kW/m2, the heat transfer coefficient variation with film Reynolds number of the lower enhanced tubes in the tube array exhibits severe undulating characteristics.

PII: S0928-1258(20)30052-8

In this study, the falling film evaporation heat transfers of R290 on an array composed of five enhancement horizontal tubes (groove tubes) are studied. The tests are performed at constant saturation temperatures 5.5 °C with change of heat flux from 10 to 40 kW/m2. The film Reynolds number ranges from 200 to 2200 and the film flow rate of refrigerant is between 100 and 660 kg/h.The results show that the film flow rate and heat flux have significant effects on R290 falling film evaporation heat transfer coefficients (HTCs) of the tubes in the tube array. With decreasing the film flow rate on the five tubes the tube HTCs display two stages, a plateau stage and a sharp drop stage. The heat transfer coefficients firstly keep more or less constant at the plateau stage and then decreasing rapidly. At the same nominal film Reynolds number the tube averaged heat transfer coefficients of tube No. 1 to tube No. 5 decrease in order of the increasing tube number from top to bottom of the array. The falling film evaporation HTC of R290 on single enhanced tube is about 4.5 times of single smooth tube HTC, and the HTCs of the enhanced tube array is higher than single smooth tube by more than 2.5 times. In addition, the R290 HTCs of the tube array are higher than those of R134a for the same tube array in the plateau region by about 25%. It is found that at high heat flux of 30–40 kW/m2, the heat transfer coefficient variation with film Reynolds number of the lower enhanced tubes in the tube array exhibits severe undulating characteristics.

Investigation on heat transfer performance of corrugated tubes in low-temperature multi-effect falling-film evaporation

ABSTRACT The enhanced heat transfer performance of falling film evaporation during desalination through altering smooth tubes by corrugated tubes was investigated. An experimental research on corrugated and smooth tubes was carried out using a horizontal-tube falling-film evaporation heat transfer experimental apparatus. The changes in heat transfer coefficient with the process parameters (sprinkling density, heat flux, heat transfer temperature difference, and evaporation temperature) were obtained according to the experimental data. The influence of tube type on liquid-film distribution outside the tube was also examined by numerical simulation. Results showed that within a certain scope increasing of any one variable of sprinkling density, heat flux, and evaporation temperature has a positive impact on the heat transfer coefficient. When the sprinkling density exceeded 0.178 kg/(m·s), the heat transfer coefficient tended to be stable. Compared with the simulation results, the liquid film coverage and thickness outside the tube significantly influenced heat transfer efficiency. When the heat flux exceeded 130kW/m2, the growth of the heat transfer coefficient significantly slowed down. Under the same experimental conditions, the heat transfer coefficient of corrugated tubes increased by 30% compared with that of smooth tubes. In addition, the content of non-condensable gas within the seawater affected the heat transfer coefficient significantly.

Heat transfer correlations of refrigerant falling film evaporation on a single horizontal smooth tube

Falling film evaporation heat transfer of R134a and its potential substitutes R290 and R600a outside a single horizontal plain tube is experimentally investigated, and the effects of the saturation temperature, film flow rate and heat flux on heat transfer coefficient are studied. Heat transfer performance of R290 is slightly superior than that of R134a, while R600a is inferior than that of R134a. The threshold film Reynolds number is determined to separate the variation trend of HTC with film Reynolds number into full wetting and partial dryout regimes. Increase of heat flux benefits the heat transfer in both full-wetting and partial dry-out regimes. New heat transfer correlations based on the present data and data for R32 and R1234ze(E) in the authors’ group are suggested for two regimes. The correlation for full wetting regime fits 96.7% of the total 542 correlated data within ±30% while fits 73.4% of the total 289 data in references from −30% to +15%, the correlation for partial dryout regime fits 97.5% of the total 162 correlated data within ±30% while fits 76.8% of the total 95 data from references within ±30%.

Falling film evaporation and nucleate pool boiling heat transfer of R134a on the same enhanced tube

Falling film evaporation and pool boiling of R134a outside a typical reentrant enhanced tube was investigated with an experimental approach. Experimental data from literature with other refrigerants were also compared. The saturation temperature was 11 °C. It was found that the overall heat transfer coefficient for the enhanced tube was as much as 3 times higher than smooth tube. Shell-side falling film evaporation heat transfer coefficient increased by a factor of 2.1–4.9. Pool boiling yielded a higher heat transfer coefficient at higher heat flux for the same enhanced tube. The dependence of falling film evaporation heat transfer coefficient on the heat flux was also different from pool boiling for the same tube. Although the bubbles in pool boiling were pushed up by buoyancy and in falling film evaporating were driven by the flow of films, the transmission of energy were both dominated by the phase change heat transfer. The contribution of forced convection to falling film evaporation heat transfer coefficient is weak compared with phase change heat transfer.

Experimental study of falling film evaporation heat transfer on superhydrophilic horizontal-tubes at low spray density

This paper presents a series of experiments concerning heat transfer in a vertical row of horizontal-tube falling film evaporator. By visualization and heat transfer experiments, the effects of hydrophilic modification on the inter-tube flow patterns and evaporation heat transfer performance on different tubes, the superhydrophilic, hydrophilic and plain tubes, were investigated. The visualization results indicated that four flow patterns, droplet, droplet/jet, jet and jet/sheet mode was achieved in this study and the hydrophilic modification has little effect on flow mode transition. Experiments have been indicated that the superhydrophilic and hydrophilic tubes were completely wetted while the plain tubes were partially wetted at low spray density. The heat transfer results demonstrated that superhydrophilic tubes could maintain a high heat transfer coefficient at a wide range of flow rates (Res from 60 to 700). While, high enhancement was achieved at low spray density, at which the plain tube was not be wetted. When Res reached 350, three surfaces had the similar heat transfer performance. Simultaneously, the heating water inlet temperature had little effect on heat transfer coefficient for superhydrophilic tubes and had a negative influence on plain tubes and hydrophilic tubes at low spray density.

Experimental study of falling film evaporation heat transfer coefficient on horizontal tube

Abstract The horizontal-tube falling film evaporation is a widely adopted technique in multiple-effect distillation desalination plant. It has a high heat transfer coefficient under quite small temperature difference. In this paper, an experimental equipment for horizontal-tube falling film evaporation was set up. Experiments were carried out to show how the heat transfer coefficient is affected by different parameters including heat flux, circumference direction of tubes, spray density, evaporation temperature, and experimental fluid. Results indicate that the heat transfer coefficient decreases after a little increase with growth of spray density. The heat transfer coefficient decreases along the tube circumference, but at the bottom of the tube, it shows increasing trend. In addition, a simple comparison between seawater and fresh water in heat transfer coefficient is also provided.

An Experimental Study of Falling Film Evaporation on Horizontal Tubes Using R-134a

AbstractThis study investigates evaporation heat transfer performance of refrigerant R-134a falling film on three horizontal copper tubes in a vertical column. Experiments were performed at saturation temperatures of 10 and 26.7°C. The liquid flows through a liquid feeder with a row of circular holes at a rate of 0.0075 ∼ 0.0363kg/ms, while heat fluxes varied from 4.5 to 48.5kW/m2. A smooth tube, a fin tube of 0.4mm fin height, 60FPI (Fins Per Inch), and a new boiling enhanced tube (mesh tube) were tested. The test results show that heat transfer coefficient of the smooth tube increases with increasing heat flux and fluid temperature, and increases slightly with increasing flow rate before dry-out occurs. At low flow rates (less than 0.015kg/ms) or when Ref (≤ 255), the fin tube is in thin film evaporation mode and results in a large heat transfer coefficient. At high flow rates (0.0225, 0.03, and 0.0375kg/ms) the falling film evaporation curves are similar to those in pool boiling. For all tubes, the fluid temperature and the flow rate have only minor influences on heat transfer coefficient before dry-out occurs. The 60 FPI tube and the mesh tube enhance the falling film evaporation heat transfer coefficient 6.3 ∼ 8.29 fold and 1.9 ∼ 5.0 fold, respectively, as compared with the smooth tube. A new correlation of falling film evaporation, accounting for contributions of nucleate boiling and spray convection, is proposed. It predicts h-values of the falling film evaporation data of the smooth surface within ±30%.

Study on heat transfer of falling film evaporation characteristics on heat pipes in negative pressure

As a high performance heat transfer method, heat pipes are being widely used in different fields of industry. Based on the boundary layer theory in the article, the rules of the velocity and the thermal boundary layers outside the heat pipe falling film evaporation in the negative pressure are discussed, and the formulas of thickness of both velocity and thermal boundary layers are given. Then based on these formulas, the calculation method of the falling film evaporation heat transfer coefficient outside the heat pipe is discussed. The effects of vacuum, falling liquid load, temperature gradation and etc. on the evaporation performance of heat transfer are analyzed. It provides the guidance in desalination application of the evaporation outside the heat pipe in negative pressure.

Heat transfer in evaporating black liquor falling film

Black liquor is a fluid with high viscosity, implying high Pr numbers. Most of the previously developed correlations for falling film evaporation heat transfer were developed for fluids with relatively low Pr numbers, and therefore are not valid for most black liquor evaporation conditions. Experimental heat transfer data from black liquor evaporation are presented here. They show that the Nu number, as expected, increases in the turbulent region. However, at a specific Re number for each Pr number level, the Nu number ceases to increase with increasing Re number. A new correlation, taking this observation into account, has been developed on the basis of experimental data in the region of 4.7 < Pr < 170 and 47 < Re < 6740, and is presented in this paper. A dry solid content dependence is included in the new correlation and improves its predictions for black liquor.

Experimental study of falling film evaporation heat transfer outside horizontal tubes

The heat transfer process of falling film horizontal evaporation includes evaporation outside tubes and condensation inside tubes, the heat transfer coefficient of the former is about 50% of that of the latter. So the overall heat transfer coefficient is influenced mainly by the falling film evaporation outside tubes. An experimental study of falling film heat transfer outside horizontal tubes was carried out in order to show how the heat transfer coefficient is affected by different parameters such as flow density evaporation temperatures, temperature difference between wall and saturation water, and mass concentration of the seawater. Experiments were conducted using 14 mm outer diameter Al-brass tubes heated by internal electric heaters so that a uniform heat flux was generated on the outside surface of tubes. The results show that when flow density Γ varies between 0.013 kg/ms <Γ<0.062 kg/ms, the heat stransfer coefficient of falling film evaporation outside horizontal tubes h increases with the increase in liquid feeding, evaporation boiling temperature and heat flux. h also increases with an increase in distributor height, however there is a maximum height in which any height above this. Besides, the amount of non-condensing gas has significant effect on h. The difference of heat transfer coefficient between freshwater and seawater is small. These results contribute to further improving the performance of heat transfer process and developing new evaporator.

Falling Films on Arrays of Horizontal Tubes with R-134a, Part II: Flow Visualization, Onset of Dryout, and Heat Transfer Predictions

In tune with the falling film evaporation heat transfer test results described in Part 1, flow visualization of the boiling process and intertube flow mode transitions from droplet to column and sheet flows have been observed, and the onset of dryout results were obtained. A new empirical approach to describe falling film evaporation with a dominance of nucleate boiling that takes into account the onset of dryout has been proposed and is applicable to plain and enhanced tubes. The method predicts most of the current local measurements for R-134a to within ±20% for conditions without dryout (the desired design condition) and has also been extended to cover conditions with partial dryout. Furthermore, a criterion for predicting the onset of dryout as a function of heat flux has also been proposed for the four types of tubes.

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/m 2 K, in the convective heat transfer range of the heat fluxes <10 5 W/m 2.