Optimization Of Foam Characteristics For Falling Film Evaporators

There is an acute scarcity in availability of water across the world. As per United Nations Development Programme (UNDP) report on Human Development almost one-fifth of the world’s population, live in areas of physical water scarcity. To combat this situation establishment of desalination plants is considered as one of the most feasible option. Multi-Effect Desalination systems are thermal based system with horizontal falling film evaporators (HTFFE). HTFFE are featured by evaporation from the liquid film over the surface of the tube and simultaneous condensation inside the tube. Condensation heat transfer coefficient is higher than evaporative heat transfer and can be improved by using heat transfer enhancement technique [1]. Metal foams which are characterized by high porosity and large volumetric heat transfer coefficient is a promising material for thermal application [2]. Numerical analysis was carried out with Aluminium foam wrapped evaporator tubes and enhancement in heat transfer was found out. Metal foam characteristics, foam thickness, porosity and permeability was optimized to give maximum heat transfer rate. Computational analysis was carried out using ANSYS Fluent 16.1. Sources terms are added using User Defined Functions (UDF) to continuity and energy equation to capture the evaporation phenomena happening from the liquid-vapor interface. Computations are carried out at saturation condition of 64.6 0C for falling film Reynolds number of range 500 to 1000. It is observed from the computational study that the heat transfer coefficient increases with decrease in the porosity and increases with the flow rate. Effect of foam thickness is not linear on the heat transfer rate, there are two competing effects. Increase in the foam thickness allow more fluid to be in contact with the metal part which tends to increase heat transfer and simultaneously increase in foam thickness will also lead to increase in the conductive resistance due to increase in falling film thickness. References: Abraham, R. and Mani, A., 2015. Heat transfer characteristics in horizontal tube bundles for falling film evaporation in multi-effect desalination system. Desalination, 375, pp.129-137. Arjun, Jayakumar and Mani, A., 2017. Computational analysis of high porosity aluminium foam for heat and mass transfer enhancement in falling film evaporator. 6th Asian Symposium on Computational Heat Transfer and Fluid Flow, Paper No. 225.

Two-dimensional Numerical Analysis on Ejector of Vapour Jet Refrigeration System

A vapour jet refrigeration system (VJRS) is an alternative to the conventional mechanically driven vapor-compression refrigeration system. The VJRS utilizes a supersonic ejector as a thermal compressor and has the potential to reduce energy consumption in refrigeration systems [1]. In the present study, the performance characteristics of VJRS ejectors with R134a as refrigerant have been investigated numerically using ANSYS Fluent. VJRS works on the principle that the high-pressure vapour from generator gets expanded through the convergent-divergent nozzle to produce high velocity stream which entrain the refrigerant vapour from the evaporator. Both the streams mix together in the mixing chamber. It results in pressure rise in the mixing chamber due to formation of shock waves followed by flow through the diffuser. The constant pressure mixing chamber where primary and entrained fluids mix together is the area of concern in the ejector. In the present work, two-dimensional analysis on the ejector geometry of VJRS (capacity = 3.5 kW) is carried out to examine the turbulent behavior and boundary layer distribution in the constant pressure mixing chamber. Also the performance of ejector at low generator temperature and low evaporator temperature has been evaluated. In addition, the present study includes the effect of area ratio and mixing chamber length of the ejector. Turbulence effects in the ejector have been modeled using the standard k-epsilon turbulence model [2]. The primary nozzle profile is developed by method of characteristics. The geometry presented here uses five different values of throat area ratio for fixed diameter of constant area mixing chamber. The exit diameter of the convergent-divergent nozzle, diameter and length of diffuser and length of constant area mixing chamber have been optimised. The ejector geometry is assumed axisymmetric and quadrilateral mesh is used for the present study. The computed results are initially validated with available literature and experimental data. For the numerical simulations on the selected ejector, generator temperature is varied from 340 K to 370 K, the condenser temperature is varied from 290 K to 310 K and the evaporator temperature is varied from 263 K to 283 K. The numerical results obtained contribute to understanding the local structure of the flow and demonstrate the role of the secondary choking in deciding the entrainment ratio. Moreover, the results help in identifying the optimum operating condition for each value of nozzle throat area ratio

Computational Analysis Of Ejector With Oscillating Nozzle

The demand for cooling increases at faster rate than the energy availability. So, refrigeration systems that use renewable energy sources need to be developed to reduce energy demand thus addressing the energy crisis and environmental pollution related to this. An ejector based refrigerator is one kind of a system which uses thermal energy sources such as solar energy, waste heat from industry, automotive exhaust, etc. to operate the system. Ejector consists of a primary driving nozzle, mixing section, and diffuser. Ejector is used to pump the refrigerant from evaporator to condenser using high pressure stream. Thus replacing compressor present in the conventional refrigeration system. Present study numerically investigates the effect of oscillation of convergent-divergent nozzle in an ejector using ANSYS Fluent.15.5. The nozzle domain will oscillate between two points in the ejector at particular frequency and amplitude. The motion of nozzle and movement of the mesh according to the nozzle oscillation is made possible by user-defined functions available in ANSYS Fluent 15.5. Variation of entrainment ratio with time has been studied at different amplitudes and frequency combination and the same is compared with the stationary nozzle. Even though the oscillating nozzle underperforms the conventional one at lower amplitude and frequency, the former one shows a trend of increasing with increasing frequency. References Selvaraju, A. Mani, Analysis of an ejector with environment-friendly refrigerants, Appl. Therm. Eng., Vol. 24, (2004). Y. Bartosiewicz, Z. Aidoun, P. Desevaux and Y. Mercadier, Numerical and experimental investigations on supersonic ejectors, Int. J. Heat Fluid Flow, Vol. 26, (2005). Arun K.M., S Tiwari and A. Mani, Three-dimensional numerical investigations on rectangular cross-section ejector. International J. Thermal Sciences, Vol. 122 (2017)

CFD Heat and Mass Transfer Studies in a R134a-DMF Bubble Absorber with Swirl Flow Entry of R134a Vapour

Study of absorber for heat and mass transfer analysis is essential to improve the performance of Vapour absorption refrigeration system(VARS). Tangential injection of refrigerant gas to liquid solution  in a bubble absorber increase the heat and mass transfer characteristics by following rotary and translation path. In this study, a vertical absorber is considered for heat and mass transfer study with refrigerant, R134a (1,1,1,2 –Tetrafluoroethane) and absorbent, DMF(dimethyl formamide). R134a vapour will be injected into the absorber using two injectors of 4.8 mm inner diameter at an injection angle of 30° to the vertical axis and parallel to the azimuthal axis of the absorber which mix well with liquid to increase the heat and mass transfer.            Finite volume method is used for the steady state with 3D cylindrical co-ordinates. Simulation is carried out for studying the heat and mass transfer behavior in laminar flow, using CFD.  For the numerical study, Mixture multiphase model is considered. A user defined function (UDF) is written for creating liquid solution mixture and refrigerant vapour to define the working fluid and its various properties like density, thermal conductivity, viscosity, etc. Grid  independent studies were carried out for the absorber geometry. Mass inlet boundary condition for solution inlet, and  vapour inlet, pressure outlet at the absorber and no-slip at wall boundary condition are used. In the absorber interface, wall constant heat flux boundary condition is defined. Evaporation and condensation model is used as phase and mass transfer interface mechanism between liquid and vapour. SIMPLER(Semi-Implicit Method for Pressure-Linked Equations) scheme is used for pressure- velocity coupling, PRESTO(Pressure Staggered Option) scheme is used for pressure and first order implicit upwind scheme is used for solving momentum equations. Converging criteria are achieved for momentum, energy and species equation by varying under relaxation factors.        Effect of solution pressure, solution flow rate, gas mass flow rate on heat and mass transfer rate, heat and mass transfer coefficients are computed. Numerical results are compared and validated with the co-axial entry of R134a in the R134a-DMF bubble absorber which show good agreement. Heat and mass transfer characteristic will be presented in this paper in terms of operational parameters

Experimental Study On Bubble Absorber With Multiple Tangential Nozzles

Vapour absorption refrigeration (VAR) system is one of the better system compared to other high grade energy utilized refrigeration system from thermodynamics point of view. For the successful operation of this system, the components are required to be designed and developed effectively and efficiently. Absorber is one of the crucial component of such a refrigeration system and bubble absorber is one type of absorber among different types of absorber. Bubble characteristics are studied experimentally with tangential nozzles, which may enhance the heat and mass transfer characteristics by following a swirling motion. Based on the inference drawn from these studies, experiments are planned with R134a and DMF combination for performance improvement of the VAR system. Visualization study of bubble growth with multiple tangential nozzles is investigated in a bubble absorber. Bubble behavior is studied with different flowing condition like still, co-current and counter-current flow of water. Bubble diameter during detachment increases with increase in gas flow rate in the above flow conditions. Effect of air flow rate, water flow rate, nozzle diameters, number of nozzles and nozzle angle with reference to vertical plane on bubble diameter is also studied. Results are compared with the available correlation in the literature which is found to be in good agreement. Comparison of performance between single and double nozzle also presented in this paper. Based on this study correlations are proposed. References: [1] M. Suresh, A. Mani, Heat and mass transfer studies on R134a bubble absorber in 134a/DMF based on phenomenological theory, International Journal of Heat and Mass Transfer 53 (2010) 2813–2825. [2] S. Ramakrishnan, R. Kumar, R.Kuloor, Studies in bubble formation-I Bubble formation under constant flow conditions, Chemical Engineering Science, 24 (1969) 731-747. [3] M. Jamialahmadi, M. R. Zehtaban , H. Muller-Steinhagen, A. Sarrafi A, J. M. Smith, Study of bubble formation under constant flow conditions, Trans IChemE, 79 (2001) Part A 523. [4] E.S. Gaddis, A. Vogelpohl, Bubble formation in quiescent liquids under constant flow conditions, Chemical Engineering Science, 41 (1986) 97-105

Falling Film Evaporation On A Thermal Spray Metal Coated Vertical Corrugated Plate Conduits

In falling film evaporation process the heat is transferred from the condensing fluid to the liquid flowing over it. Falling film types of evaporators are widely used in refrigeration, desalination, petroleum refining, chemical industries, etc. Compared to flooded type evaporators, falling film evaporators need less amount of refrigerant and will give higher heat transfer rates even at lower heat fluxes. Tube geometry and tube size have an important role on the performance of the falling film evaporators. Geometry of the tube can be varied by heat transfer enhancing techniques and also by changing the shape of the tube. Thermal spray metal coating is a heat transfer enhancement technique which is done by spraying molten metal on any heat transfer surface. Vertical plate with sinusoidal corrugation is a surface enhancement technique which can be cheaper, compact and lighter for falling film evaporation compared to shell and tube configuration. This paper presents a two dimensional CFD study of water falling film evaporation on a thermal spray metal coated vertical corrugated conduits. Two-phase flow simulation is done by using a finite volume method based commercial software, with turbulence modeling which is done by k-Â two equations with shear stress transport (SST). Sinusoidal corrugations with different porosity have been selected for the study. Evaporation and heat transfer during falling film evaporation are included through user defined functions (UDFs). Effect of Reynolds number (Re), wall superheat and surface roughness on heat transfer coefficient is presented. Surface roughness is varied from 0.33 to 4.73. Re is varied from 500 to 2000 for two Prandtl numbers 2.9 and 3.4. Wall super heat is varied from 2K to 10K. The numerical results are compared with the results of horizontal circular tube falling film evaporation from literature. An enhancement of film heat transfer coefficient is observed for the vertical corrugated plate conduits

Performance Evaluation Of Bubble Absorber Using R134a/DMF In Vapour Absorption Refrigeration System With Swirl Injector

Vapour absorption refrigeration systems (VARS) importance has increased due to utilization of solar energy, waste heat, etc. To improve the efficiency of this system, it is necessary to increase the heat and mass transfer in the absorber with tangential injectors. Experiments were carried out to analyze a tubular heat exchanger used as an absorber with tangential injector. In this study, a vertical bubble absorber is considered for heat and mass transfer studies with refrigerant, R134a (1,1,1,2 –Tetrafluoroethane) and absorbent, DMF(dimethyl formamide). R134a vapour is injected into the absorber using two injectors of 4.8 mm inner diameter at an injection angle of 30° to the vertical axis and parallel to the azimuthal axis of the absorber, which enhances mixing of R134a with liquid R134a-DMF solution to increase the heat and mass transfer. Experiments were carried out by varying the operational parameters by varying refrigerant flow rate from 5 lph to 15 lph and liquid solution flow rate in the range of 20 lph to 50 lph. Chilled water is used to load the refrigeration system evaporator with flow rate 100 lph and temperature in the range of 5 to 20oC. Hot water is supplied to the generator in the range of 60 to 91oC with flow rate of 80 to 200 lph. Cold water is used as coolant to the condenser and absorber in the temperature range of 11 to 25 oC with the flow rate of 80 to 160 lph. Effects of operational parameters viz., gas flow rate, solution initial concentration, and solution pressure and solution temperature and number of injector on absorber performance are varied and analysed. Mass transfer effectiveness, heat transfer effectiveness, overall heat transfer coefficient, volumetric mass transfer coefficient, absorption rate and heat rejected from the absorber have been evaluated for the absorber. Heat and mass transfer coefficients evaluated from the experiments are compared with numerical model and it is found to be in good agreement. REFERENCE Suresh M., Mani A. (2012), Experimental studies on heat and mass transfer characteristics for R134a-DMF bubble absorber, International Journal of refrigeration 35 pp. 1104-1114. Santosh Kumar Panda, Mani A. (2014), Bubble dynamics study with tangential nozzles in a bubble absorber, International Sorption Heat Pump Conference at Maryland University, Maryland,USA

Three-dimensional Numerical Investigations on Ejector of Vapour Jet Refrigeration System

Vapour Jet Refrigeration System (VJRS) is preferred among various heat operated refrigeration systems because it has potential of utilizing low temperature heat source, consumes less electrical energy causing less atmospheric pollution and requires less maintenance cost due to absence of moving parts [1]. Ejector, a thermal compressor, is one of the crucial components of VJRS. Performance of the whole system is based on the satisfactory operation of the ejector. Hence, it has to be designed carefully and effectively to realize good performance. Key criteria for enhancing the performance of the ejector demand better mixing of the primary and secondary streams. This would result in better momentum exchange between the two streams which in turn increases the entraining performance and compression ratio. The present work focuses on enhancing the performance of the ejector using *three dimensional CFD analysis with R134a by introducing _swirl_ in the primary stream of ejector*. In literature, compressible and axisymmetric swirl flow ejector working with steam has been investigated [2]. The novelty of the present work is the three-dimensional analysis of swirl flow ejector working with the refrigerant R134a. The flow in the ejector is considered to be steady, compressible and turbulent in nature. The flow domain contains two inlets viz., primary and secondary, and one outlet. Constant pressure boundary conditions are applied for the two inlets and the outlet. Primary inlet condition is given slightly superheated to avoid the condensation of the refrigerant at the exit of the primary nozzle due to expansion of the primary stream in the nozzle [2]. The turbulence model for this study is chosen as k-? model as used by [1]. Real gas thermodynamic behaviour and transport properties of R134a are obtained using ANSYS Fluent through* \u27NIST Real Gas Model\u27 which uses REFPROP subroutines*. Validation has been carried out by comparing the computed results of ejector without swirl with the experimental results from literature. Shock patterns, Mach number, radial velocity and tangential velocity variations along the ejector at various swirl angles are studied for a particular design operating condition. *Optimum swirl angle* which gives maximum entraining performance has been obtained. The computed results of ejector with swirl are compared with the ejector without swirl for the same operating conditions. Results indicate that the entrainment ratio gets improved for the ejector in presence of swirl as compared to the ejector with no swirl. Thus, the presence of swirl is expected to improve the coefficient of performance of VJRS

Bubble Dynamics Studies In An Absorber With Swirl Entry Of Absorption Refrigeration System

Experimental investigations have been carried out on a glass bubble absorber with swirl entry of air to visualize air bubble characteristics. Experiments are carried out in still, co-current and counter-current flow direction of the water. In bubble visualization studies, it was observed that the bubbles forms at the swirl generator surface, grows and detaches the swirl generator surface and moves upwards in water. Effect of air and water flow rate on the bubble detachment diameter was studied. Bubble detachment diameter always increases with air flow rate irrespective of water flow direction. Bubble diameter increases in counter-current flow of water, whereas it decreases in co-current flow of water

Measurement Of Film Thickness And Temperature On Horizontal Metal Spray Coated Tube Falling Film Evaporator Using Interferometric Techique

’Water’ is the ’Essence of Life’. It is an irreplaceable precious resource that is core of life on earth, a vital commodity that is critical for human survival, socio-economic developments and for the preservation of a healthy ecosystem. Current trends indicate that two-thirds of the world’s population will be living in water-stressed countries by 2025(wat (2006)). In order to eradicate or to provide sufficient water requirement for mankind desalination plays a pivotal role. This paper presents studies on horizontal tube falling film evaporator for Multi effect desalination (MED) system with spray coated tubes. The most important component in any MED system is the falling film evaporator. The wide acceptance for this kind of evaporators is because of the fact that it is characterized by a very low-pressure drop. In MED systems, falling film evaporation takes place outside the tube geometry utilizing the latent heat of condensing vapour inside the tube. Convective evaporation, as well as low-temperature nucleate boiling, occur in the film as it flows over the tube depending on the operating conditions(Abraham and Mani (2015)). The liquid falls on the top of the tube and flows down along the curved tube surface. There is a phase change on both sides of the tube and the evaporation outside the tube helps vapour to be separated from the liquid as soon as it is formed. Two different tubes surfaces were studied, namely bare copper tube and copper tube coated with alloy of Al2O3 and TiO2. Scanning electron microscope, Energy-dispersive X-ray spectroscopy, 3D surface profilometer were utilized to study about the surface texture, composition and to find surface roughness values attributed with each tubes. An optical shadow method (non-intrusive) incorporating Otsus’s algorithm was used to evaluate the film thickness around the circumference of the cylinder, and a Mach-Zehnder Interferometer (MZI) was employed to visualize the isotherm formation (Maliackal et al. (2021)). All studies were performed for complete wetting of the tube. The measured film thickness was compared with commonly used empirical formulas. Further, the effectiveness of using those empirical formulas for small diameter tubes was analyzed. A novel interferometric technique was used to analyze the film interface temperature, and a comparative study was performed for the two different tube geometries. A standard error of mean (SEM) analysis was performed on every data set