Vertical Evaporator Research Articles

"Heat Transfer and Fluid Flow in Falling Film of Vertical Tube Evaporation: a Numerical Solution".(Dept.M)

The present work studies the heat transfer and fluid flow in a falling film of vertical tube evaporators numerically. A finite difference technique, with iterative method, has been applied in these solutions, The flow is considered turbulent, cylindrical coordinates, and steady. Momentum, and energy equations are formulated in finite difference technique and computations are performed, utilizing Gauss-Seidal point iterative method to obtain an acceptable accuracy. Velocity developed and temperature distributions in a falling film have been obtained, and the heat transfer coefficient is calculated. The results have been compared with those obtained by others and a good agreement hag been found.

Investigation on a small‐scale vertical tube evaporator multieffect desalination system: Modeling, analysis, and optimization

AbstractA growing population with depleting water resources has increased the requirement for desalination systems. Large‐scale desalination plants have seen a growth in the recent period; however, the small‐scale (SS) decentralized desalination plants' need has not been realized for the rural population. Low specific heat consumption for multieffect desalination systems makes it suitable for such decentralized operation. The challenge now lies in determining the system capacity and optimal operational range for the SS requirements. In this study, the thermoeconomic model for an SS multieffect desalination system for various configurations is developed. Optimization of the SS plant for the number of effects is performed to determine the optimal operational range of motive steam pressure, motive steam flow rate, and feed water flow rate. Total distillate production and freshwater cost are focused on objectives and constraints imposed over the input parameters with SS production. The results reveal that for a distillate production of 750 L/day, the motive steam flow requirement is estimated to be 25–35 kg/h with a pressure range of 2–5 bar. This study provided an overview for selecting the number of effects based on the commercial aspect of total production requirements.

Optical Analysis of a New Solar Distiller with Cylindrical Surface Concentrator and Vertical Gap Evaporator

In this paper, a new solar distiller floating on ocean with cylindrical surface concentrator and vertical gap evaporator is proposed for solving the problem of freshwater shortage in islands. When the distiller is floating on ocean, the vertical gap will fill with seawater automatically due to the siphon effect of hydrophilic material. Then the seawater is heated to generate vapor when the incidence sunlight is concentrated to the gap by the cylindrical concentrator. Finally, the vapor reaches the arched transparent glass at the top of the device and condenses to produce fresh water. Optical simulation for the solar distiller is carried out to find the optimal radius of the cylindrical concentrator and the height of the vertical gap. The results indicate that when the radius and height is 6 cm and 5 cm respectively, 80% of the sunlight number within the incidence angle of 45° can be captured by the seawater in the vertical gap. The annual optical performance of the distiller is analyzed for the region within 17° north latitude. As the result, the device placed in east-west direction possesses superior performance. There are more than 10 working hours and 5 working hours in which the reception rate is more than 80% in summer solstice and winter solstice respectively. In autumnal equinox, there are more than 11 working hours with the reception rate exceeding 90%. Energy balance analysis for the whole system is carried out and the stable evaporation rate per unit solar collector area increases gently from 0.12 g/ (m2‧s) to 0.65 g/(m2‧s) when the solar irradiance increases from 500 W/m2 to 1000 W/m2.

Investigation of Refrigerant R134a Two Phase Flow Heat Transfer in Vertical Heat Exchanger Channel

Two phase flow boiling heat transfer and pressure drop in the vertical evaporator tube section of refrigeration system have been experimentally investigated using refrigerant R134a as a working fluid. The objective of the present work is to investigate experimentally the effect of heat flux, mass flux, vapor quality and saturation temperature on refrigerant flow boiling heat transfer characteristics in the evaporator of refrigeration system. These investigated parameters have significant impacts to enhance the thermal performance of the evaporator. The experimental investigations were conducted in smooth copper tube with inner diameter 5.8 mm and 600 mm length under different test conditions. The test conditions considered in this study were, for heat flux of 7.718-32.78 kW/m2, mass flux of 97.3-148.7 kg/m2.s , saturation temperature of -20.58 to -15.68 ?C and vapor quality of 0.3-1. It can be concluded from the results that, the average heat transfer coefficient at relatively greater mass flux 148.7 kg/m2.s was higher in range of 21% compared to other mass fluxes at constant test conditions. The relatively higher value of heat transfer coefficient was observed at heat flux 32.78 kW/m2 with average increase of 19% compared to the relatively lower value 23.38 kW/m2. The enhancement in local heat transfer coefficient at saturation temperature -15.68?C was higher by about 12% than that for the relatively lower temperature -20.58?C. The effect of increase in mass flux and heat flux on pressure drop in the evaporator tube was about 9% and 7.5% respectively.

P‐49: Ultimate Solution: The Belt Plane Source Evaporators for Future AMOLED and QD OLED TV

There are two type evaporators such as cluster and inline equipped with linear sources in OLED manufacturing. The inline type evaporator has been used for WOLED and QD‐OLED and there is a glass substrate bending problem in 10G size manufacturing. The cluster type evaporator has been used for AMOLED and there is a shadow distance limitation to improve the display resolution. The belt plane source FMM evaporator and the vertical belt plane source evaporator equipped with three dimensional shape plane sources will be used for very high ppi (pixel per inch) AMOLED and 10G OLED TV device, respectively. They are the ultimate evaporator‐solutions.

Гидравлическое сопротивление при кипении хладагентов в трубах горизонтальных и вертикальных испарителей судовых холодильных машин

The article presents the comparison of the head losses under the boiling refrigerant flow in horizontal and vertical evaporators. The values of all the components of the hydraulic resistance, such as the friction losses, local resistance, flow acceleration and leveling head have been calculated, compared and analyzed. Graphic dependences of the pressure loss of the two-phase flow on the mixture density and the apparatus height, as well as the effect of hydraulic resistances on the changes in boiling temperature have been obtained. It has been stated that studying the effectiveness of vertical evaporators comprises two main components: defining heat transfer and hydraulic resistance in the course of working fluids boiling in the vertical pipes. The second part of the study is under consideration. Vertical evaporators are higher than horizontal ones, which leads to greater losses when overcoming the leveling head of a larger column of a two-phase flow. Other components of the hydraulic resistance in the horizontal and vertical apparatus are comparable, although the friction losses and local resistance losses in the vertical evaporators may be less. In a vertical apparatus the main losses appear due to overcoming the leveling head, the remaining losses are smaller than in the horizontal apparatus. Therefore, when designing vertical apparatuses, one must not use the ratio of the apparatus length to its diameter, which is recommended for the horizontal apparatuses (L/d ratio for horizontal and vertical is 6). The ratio for the vertical apparatus should be significantly less. The experiments performed on the air-water model proved that L/d ratio of vertical evaporators should be within 1.0 ÷ 2. More precise values of this parameter can be found after testing R410A refrigerant.

Performance comparison of a vertical direct expansion geothermal evaporator: Part I, single U-pipe using different refrigerants

Abstract CO2 is a promising candidate for replacing synthetic refrigerants used in direct-expansion ground-source heat pumps (DX-GSHPs). The present paper, as the first part of two, presents a numerical performance comparison of a vertical single U-pipe direct expansion geothermal evaporator using different refrigerants; namely R744, R410A, R22, R407C, R12345yf, and R134a. Each refrigerant was tested through a borehole heat exchanger (BHE) performance simulation at a constant rate of heat extraction. Operating parameters were chosen so that each refrigerant is subjected to a complete evaporation cycle, and a similar pressure drop from the inlet to outlet of the borehole. Several BHE parameters such as fluid temperature, pressure, mass flow rate, required pipe dimensions, and required power for fluid circulation were evaluated for different refrigerants. Finally, the overall performance of each refrigerant was characterized by the heat exchange capacity for a unit of mass flow rate and pipe surface area. Results showed that CO2 offers unique characteristics favoring the design of compact BHEs, as it required the smallest mass flow and pipe surface area. It is also concluded that the performance of a BHE with CO2 can be significantly improved through using unconventional configurations. This will be discussed further in the second part (PART II).

Performance comparison of a vertical direct expansion geothermal evaporator: PART II, multiple U-pipes

Part I showed that Carbon Dioxide (CO2) is a promising candidate for use in vertical direct expansion ground evaporators, as it requires smaller pipe diameters within the geothermal heat exchanger and lower power for refrigerant circulation compared to conventional refrigerants. Furthermore, pressure change along the geothermal evaporator influences marginally the evaporation temperature leading to quasi-isothermal evaporation of CO2. However, in single U-pipe configurations, small pipe diameter means lower pipe surface area and therefore higher thermal resistance. In Part II, all these advantages are taken into consideration toward proposing new geothermal evaporator configurations with multiple U-pipes (2 to 4 U-pipes in one borehole). R410A is also compared against CO2 for the new configurations. Operating parameters of all configurations were chosen so that CO2 and R410A are both subjected to a complete evaporation for equal total pipe surface area and equal heat extraction rate. Although all configurations have same total pipe surface area, it is shown that adding U-pipes and reducing pipe diameters diminishes borehole thermal resistance significantly. A novel configuration is also proposed to lower pressure drop of CO2 in the 4-U borehole configuration by insulating a portion of the downward U-pipe legs. Decreasing pipe dimensions by adding U-pipes while keeping total pipe surface area constant offers an opportunity to bundle pipes closer to each other and make the evaporator more compact as seen in the proposed Polymer Block configuration.

Heat Transfer Performance of Vertical Tube Falling Film Juice Evaporator and a Sensitivity Analysis

Abstract Fresh fruit juice has a short storage period and it usually needs to be concentrated for shelf-life extension. Evaporation of fruit juice is an attractive method and vertical tube falling film evaporator is one of good choice. The main problem in the design of the evaporator is the reliable estimation of the heat transfer coefficient for all effects. In this paper, the effects of feed flow rate, evaporation temperature, temperature difference, and juice soluble solid content on heat transfer coefficient of a vertical falling film evaporator were investigated. The Bayesian treed Gaussian process model was used to identify the key variables affecting the heat transfer coefficient, which showed that evaporation temperature had the greatest influence on the sensitivity of heat transfer coefficient. It also demonstrated that complex interactions existed between the four operating parameters had a significant effect on the overall heat transfer coefficient.

Theoretical and experimental analysis of a vertical direct expansion geothermal evaporator using CO2 as refrigerant

In direct expansion ground coupled heat pumps (DX-GCHPs), the borehole heat exchanger (BHE) plays a major role, as it is the component where heat transfer between the ground coupled heat pump (GCHP) and the soil occurs. To address the current challenges of using CO2 as refrigerant in DX-GCHP systems, theoretical and experimental investigations were performed on a single U-tube vertical direct expansion BHE using CO2 as the refrigerant. First, an experimental setup of a transcritical CO2 DX-GCHP built at CanmetENERGY laboratories was used to monitor, collect, and analyze data for refrigerant flow, pressure drop, and ground temperature along the boreholes. The CO2 evaporation process in the BHE for different mass flow rates per borehole was then experimentally analyzed. Later, a previously developed theoretical model of the BHE was validated using a set of experiments collected with the current test apparatus. Finally, the model was used for the analysis of the effect of the following parameters on the performance of the BHE: (i) the refrigerant flow rate, (ii) the borehole wall temperature and average fluid temperature over the test period during the evaporation process, (iii) the pressure drop, (iv) the required power for CO2 circulation, and (v) the evaporating temperature change in the BHE.

Heat Transfer Coefficients Analysis in a Helical Double-Pipe Evaporator: Nusselt Number Correlations through Artificial Neural Networks.

In this study, two empirical correlations of the Nusselt number, based on two artificial neural networks (ANN), were developed to determine the heat transfer coefficients for each section of a vertical helical double-pipe evaporator with water as the working fluid. Each ANN was obtained using an experimental database of 1109 values obtained from an evaporator coupled to an absorption heat transformer with energy recycling. The Nusselt number in the annular section was estimated based on the modified Wilson plot method solved by an ANN. This model included the Reynolds and Prandtl numbers as input variables and three neurons in their hidden layer. The Nusselt number in the inner section was estimated based on the Rohsenow equation, solved by an ANN. This ANN model included the numbers of the Prandtl and Jackob liquids as input variables and one neuron in their hidden layer. The coefficients of determination were for both models. Both ANN models satisfied the dimensionless condition of the Nusselt number. The Levenberg–Marquardt algorithm was chosen to determine the optimum values of the weights and biases. The transfer functions used for the learning process were the hyperbolic tangent sigmoid in the hidden layer and the linear function in the output layer. The Nusselt numbers, determined by the ANNs, proved adequate to predict the values of the heat transfer coefficients of a vertical helical double-pipe evaporator that considered biphasic flow with an accuracy of ±0.2 for the annular Nusselt and ±4 for the inner Nusselt.

Experimental investigation of flat plate closed loop pulsating heat pipe

Pulsating heat pipes (PHPs) have promised to be effective heat spreaders. From the fabrication perspective, the PHPs are less intensive than their conventional counterparts. Despite the advantages, there still exists shortage of data in both experimental and analytical domains towards reliable design and prediction of PHP performance. In this study, a flat plate closed loop PHP has been experimentally investigated for its thermal performance. The PHP with 12 channels, each 2.2 mm deep × 2.0 mm wide, was tested with deionized water for a fill ratio of 70% by volume for various orientations starting from vertical evaporator below condenser (90°) to near horizontal (7.5°) orientation for a single heat load of 50 W to calibrate the test setup against published literature. The PHP was also tested for methanol with various fill ratios (30, 40, 50, 60 and 70%) for 50 W. The best performing fill ratio of methanol was tested for various heat loads (10 to 100 W) for the vertical and the near horizontal orientation. An attempt has been made to resolve the critical angle after which the PHP ceases to perform when tilted towards horizontal orientation. The PHP performance as expected was best at the 90° (vertical) orientation with very little deterioration up to 45°. The thermal resistances were estimated for two fluids, for various orientations, heat loads and fill ratios. The results indicate that at near horizontal orientation methanol performed better than water for 70% fill ratio. The 40% fill ratio of methanol yielded the least thermal resistance for all orientations. Also the thermal resistance of the PHP decreased with increasing heat load for different orientations. The PHP operated successfully up to 7.5–10° below which the PHP was observed to have dry out.

Heat transfer study of PVDF hollow fiber heat exchanger for desalination process

The polymer hollow fiber heat exchangers have been of great interesting because of their low investment, lightweight and corrosion resistant features in comparison with metal heat exchangers. This paper is focused on heat transfer process with and without phase change by experimental investigations of poly(vinylidene fluoride) (PVDF) hollow fiber heat exchangers. The maximum overall heat transfer coefficient (OHTC) in the experiments reached up to 612 W/(m2·K) for liquid-liquid system, 883 W/(m2·K) for condensation system and 1168 W/(m2·K) for condensation/evaporation system. The phase change remarkably improved the heat transfer performance. With respect to experimental results, a new correlation was developed to predict the condensation heat transfer coefficients. Furthermore, the thermal efficiency of the vertical tube evaporator achieved up to 92%. The comparison of the present hollow fiber heat exchanger with other desalination technologies suggested that the superior performances of high efficient, compact, anti-corrosion and anti-fouling made this technology more prospect in desalination field.

New void fraction equations for two-phase flow in helical heat exchangers using artificial neural networks

In this research, two new empirical equations based on Artificial Neural Network (ANN) were developed to determine the new void fraction in two-phase flow inside helical vertical coils with water as work fluid. The first model included vapor fraction (xg), density ratio ρgρl, viscosity ratio μlμg, and curvature ratio dD, as input variables, and 2 neurons in the hidden layer to predict satisfactorily the void fraction. In order to simplify the model, a second model of ANN was proposed without curvature ratio. The best architecture to the second model, with 3 input variables, was also with 2 neurons in the hidden layer. The coefficients of determination were R2 > 0.9 to both models. The ANN models of void fraction satisfied the interval condition of 0–1. Therefore, both models have been considered to be satisfactory for predicting the behavior of void fraction of a two-phase flow. To validate these new void fraction equations, three different helical heat exchangers described in previous works reported, were applied in two ways: first, experimental and simulated heat fluxes were compared using steady state test data from two helical double-pipe vertical evaporators integrated into two absorption heat transformers; second, experimental and simulated heat fluxes were also compared in an innovative design prototype full-scale helically coil steam generator in which, numerical results for pressure along the tube reveal a better way to represent the two-phase flow. The second evaluation also provided evidence on the successful extrapolation of simple ANN equations of void fraction in function of dimensionless numbers. The analyses of the contribution of input variables in the ANN model showed that the curvature ratio could not impact the simulative accuracy of void fraction under the experimental conditions worked.

Performance Analysis of Vertical Evaporator Refrigerator with Refrigerant R134a and R600a

Performance Analysis of Vertical Evaporator Refrigerator with Refrigerant R134a and R600a

Model development of heat/mass transfer for internally cooled dehumidifier concerning liquid film shrinkage shape and contact angles

Moisture affects building materials, the thermal comfort of building occupants and the work performed by them. The plate liquid desiccant air-conditioning system (LDACS) is a promising dehumidification alternative to traditional air-conditioning system for lower energy consumption and less pollution. The shrinkage of the falling film on working plates critically influences the dehumidification performance by affecting the wetting area and film thickness. This paper developed a new model of plate dehumidifier concerning the shrinkage shape and the variable film thickness of falling film. The new model was validated by experiments and compared with existing models. The results indicated that the effect of contact angles on dehumidification performance can be accurately predicted. The moisture removal rates increased rapidly from 2.0 g/kg to 2.56 g/kg as the contact angles decreased from 85° to 5°, while the wetting area increased from 0.145 m2 to 0.176 m2. The distribution of the humidity ratio of process air along flow direction with different contact angles was also simulated. Besides, the average film thickness decreased from 0.952 mm to 0.889 mm as the contact angles decreased from 85° to 5°. The annual electricity consumption of plate LDACS with different contact angles for a typical building in Hong Kong was estimated and analysed by using the newly developed model. The new model can achieve a better predictive accuracy by considering the exact shrinkage shape of falling film and the findings can provide a new insights improving the performance of plate dehumidifiers and other industrial applications, such as vertical condensers, evaporators and absorption towers.

Air-Cooled Thermosyphon for Press-Pack Stack of Semiconductors

Medium voltage converters make extensive use of press-pack semiconductors in several products ranging from drives to rectifiers. Better cooling brings significant advantages: the possibility of drawing more power using the same semiconductors and thus effectively extending the converter operating range, a higher reliability, and cost savings on semiconductors for equal converter power. In this article, the experimental characterization of a baseplate to air thermosyphon for the double-side cooling of press pack thyristor modules in presented. Effects of refrigerant fluid filling, heat load, air temperature and air flow on the junction temperature were studied in both vertical and horizontal stack configuration. Vertical evaporator yields the best performance with 120°C junction temperature at 40°C air and 30W/cm2 heat loss.

Passive Control of Two-Phase Flow Thermal Instabilities in a Vertical Tube Evaporator

Passive heat transfer techniques are considered to be one of the most important means to enhance heat transfer in heat exchangers that allow also reducing their size and manufacturing cost. Moreover, this passive technique can also be used to control the thermal instabilities caused by the two-phase flow in the evaporators. The thermal instabilities are undesirable because they can lead to a tube failure. For this purpose, a numerical study of the two-phase flow with evaporation in a vertical tube has been performed in this work. The volume of fluid (VOF) multiphase flow method has been used to model the water vapor–liquid two-phase flow in the tube. A phase-change model, for which source terms have been added in the continuity and energy equation, has been used to model the vaporization. The numerical simulation procedure was validated by comparing the obtained results with those given in the literature. The passive control technique used here is a ring element with square cross section, acting as a vortex generator, which is attached to the tube wall at various positions along the tube. Instabilities of temperature and void fraction at the tube wall have been analyzed using fast Fourier transforms (FFTs). The results show that the attachment of the control element has a significant influence on the value and distribution of the void fraction. Higher positions of the control element along the tube allow reducing the magnitude of void fraction oscillations.

Empirical correlations for the determination of R134a's convective heat transfer coefficient in horizontal and vertical evaporators having smooth and corrugated tubes

Boiling heat transfer of R134a flowing in the inner tube and hot water flowing in the annulus were examined inside smooth and corrugated tube-in-tube heat exchangers in authors' previous studies experimentally. The apparatus has 0.85m long double tube for vertical orientation and 2.5m long double tube for horizontal orientation as its test sections whose inner ones are smooth and corrugated copper tubing having inner diameters of 0.0087m for vertical tubes and 0.0081m for horizontal tubes. The range of mass fluxes are 20,300 and 400kgm−2s−1 for vertical tubes, and 300, 400 and 500kgm−2s−1 for horizontal tubes. The average vapor qualities vary from 0.14 to 0.86 for vertical tubes, 0.09–0.81 for horizontal tubes, and saturation pressure interval is between 4.5 and 5.7 for both tube orientations. The average boiling heat transfer coefficient of R134a is calculated by means of the energy balance in the test section. In this existing study, the predictability of almost whole well-known empirical correlations in literature suggested for convective boiling flows in smooth and corrugated tubes is experienced by using the authors'large number of experimental database (302 data points for vertical tubes and 227 data points for horizontal tubes). Characteristics of trend lines plotted for the change of vapor quality, saturation temperature, mass flux and heat flux with experimental boiling heat transfer coefficients are examined as well in relation to various saturation temperatures and mass fluxes. Furthermore, the most successful correlations validated their predictabilities for both vertical and horizontal oriented evaporators having smooth and corrugated tubes.

Modelling of a cross flow evaporator for CSP application: Analysis of the use of different two phase heat transfer and pressure drop correlations

Heat exchangers consisting of bundles of horizontal plain tubes with boiling on the shell side are widely used in industrial and energy systems applications. A recent particular specific interest for the use of this special heat exchanger is in connection with Concentrated Solar Power (CSP) applications. Heat transfer and pressure drop prediction methods are an important tool for design and modelling of diabatic, two-phase, shell-side flow over a horizontal plain tubes bundle for a vertical up-flow evaporator. With the objective of developing a model for a specific type of cross flow evaporator for a coil type steam generator specifically designed for solar applications, this paper analyzes the use of several heat transfer, void fraction and pressure drop correlations for the modelling the operation of such a type of steam generator.The paper after a brief review of the literature about the available correlations for the definition of two-phase flow heat transfer, void fraction and pressure drop in connection with the operation of steam generators, focuses attention on a comparison of the results obtained using several different models resulting by different combination of correlations. The influence on the analysis of the performance of the evaporator, their impact on significant design variables and the effective lifetime of critical components in different operating conditions, simulating the daily start-up procedures of the steam generator is evaluated. The importance of a good calibration of the model based on the comparison with some experimental data is recognised.