Falling Film Generator Research Articles

Thermodynamic performance and heat and mass transfer analysis of air source absorption heat pump for heating

To lower CO2 emission and obtain better heating efficiency compared with conventional gas boilers, this paper proposed a solar air source absorption heat pump (SAAHP) with falling film heat exchanger for district heating. The thermodynamic model is established to analyze the performance of the proposed system. The falling film generator as the key heat exchange component has been designed carefully, and then the numerical models of the falling film generation process which include one and two-dimensional numerical models are developed. According to the thermodynamic analysis, the primary energy efficiency (PEE) and heating capacity of the system are 1.65 and 45.02 kW at the generation temperature of 190 °C and ambient temperature of −7 °C, respectively, which can save 41.69 t of CO2 emissions from heating. The two-dimensional numerical model indicates the main heat and mass transfer occur between the falling film height x = 0 mm and x = 200 mm, and the interface heat transfer decreases from 74.6 kJ/(m2·s) to 40.5 kJ/(m2·s) in a typical vertical falling film generation process. In addition, the energy transfer as the inter-diffusion is corrected in the energy equation which improves the prediction accuracy by about 1.5% of the two-dimensional model. The research provides a reference for the optimization design of the generator, where one-dimensional models can quickly guide the design and selection of equipment, modified two-dimensional numerical models and Computational fluid dynamics (CFD) models can provide references for the refined optimization design, which attributes to the improvement of thermodynamic performance of the solar absorption heat pump system.

Heat and mass transfer characteristic of vertical falling film generator with annular structure for ammonia-water system

Ammonia–water solution is widely used in absorption cycle systems, but the generation of ammonia from solution requires a great amount of heat consumption. In this work, a vertical-tube heat exchanger was adopted as the generator of an ammonia–water absorption cycle. The generation of ammonia from an ammonia–water solution in the generator was experimentally analyzed. The generator has an annular structure: a falling film of ammonia–water solution flows inside the inner tube, and an upflowing water stream flows in the annular space between the outside surface of the inner tube and the inside surface of the outer tube. First, the heat transfer enhancement effect of the annular structure was confirmed using water as the working fluid; then, the effect was tested for an ammonia–water solution. It was found that the heat transfer coefficient of Annulus I (0.5 mm annulus gap) was 316.2% higher than that of Annulus III (2.5 mm annulus gap) at the water flow rate of 50 g⋅s−1 through the annulus space. When the flow rate of the upflowing fluid exceeds a certain value, the effect of heat transfer for these annuli deviates obviously. The solution in the falling film with an initial ammonia concentration of 18.7 wt. % was used in the generation experiment. The outlet ammonia concentration of Annulus I was 12.7% lower than that of Annulus III at the water flow rate of 33.3 g⋅s−1. Based on the Sieder–Tate formula, the Nusselt number correlation equations were developed with ±10% error.

A detachable plate falling film generator and condenser coupling using lithium bromide and water as working fluids

A novel detachable plate falling film heat and mass exchanger (HMX) coupling using lithium bromide and water as working fluids is proposed, with both maintainability and compactness taken into account and a prototype built. The HMX coupling can be applied as either a generator and condenser coupling, or an absorber and evaporator coupling in an absorption heat pump using water as refrigerant. The prototype, working as generator and condenser coupling, was evaluated and analyzed experimentally. In a sensitivity study of the operating variables of generator, heat transfer coefficient of the generator varied between 0.345 kW/ (m2·K) and 0.660 kW/ (m2·K), while the mass transfer coefficient of the generator was between 2.7 × 10−5 m/s to 7.8 × 10−5 m/s. The heat transfer coefficient of the condenser varied from 0.627 kW/ (m2·K) to 0.731 kW/ (m2·K), and the minimum generation rate of the designed condenser to maximize its heat transfer capability is 3.85 g/s.

Heat transfer characteristics of a falling film generator for various configurations of heating tubes in an absorption chiller

A generator is a heart of an absorption refrigeration system. It plays a significant role in the operation cycle by desorbing the water vapor from a lithium bromide (LiBr) solution. The wettability of the solution over the heating tubes has been considered for heat transfer enhancement in a water-LiBr falling film generator. This paper proposes seven types of heating tube bundles modified by surface treatments to improve the wettability. Each heating tube bundle was installed in the falling film generator for each experimental set, and the heat transfer characteristics were observed to evaluate the shell side heat transfer coefficients. Experiments were carried out with the following operating conditions: feed solution temperature (63.5 °C), solution flow rate (4–9 kg/min), feed solution concentration (55–58%), and pressure at the shell side (4.7–6.7 kPa). Hot water was used as a heat source, and the operating parameters were the feed temperature (85–97 °C) and the flow rate (10–28 L per minute). Each heating tube bundle had entirely different heat transfer characteristics, which were distinctly illuminated. The experimental correlations for a shell side heat transfer were developed to estimate the real benefits of using the proposed heating tube bundles. All the obtained empirical correlations were validated within an error limit of around ±20%.

Experimental study on LiCl solution falling-film generation process outside a vertical tube

For the purpose of the development of a generator with higher performance utilizing LiCl/H2O as the working pair in a two-stage absorption refrigeration system, an experimental investigation of LiCl solution falling film generation outside a vertical tube as the high-pressure stage solution cycle was conducted. An experimental setup was established to study heat and mass transfer process in a falling-film generator for LiCl/H2O using low-temperature heat source. The experiments showed effect of the flow rates of hot water and falling film solution, temperature of heat source, concentration of the solution and generation pressure on the LiCl/H2O generation process. To compare with conventional LiBr/H2O solution, some comparative experiments of falling film generation were tested in order to discuss the mass transfer performance of two different working pairs in a high-pressure generator. The results showed that the two working pairs had similar mass transfer rate. The LiCl/H2O solution had a similar mass transfer rate to that of the LiBr/H2O solution, while the concentration of LiCl/H2O solution was much lower with smaller circulation ratio of the solution, and it could make for possible improvement in the thermal performance of the absorption refrigeration system.

Development of a compression-absorption heat pump system for utilizing low-temperature geothermal water

Heat pump is an effective way to use the low temperature geothermal water with temperature lower than 50℃ for building heating. Compared to the conventional vapour compression heat pump, the compression-absorption heat pump (CAHP) can obtain higher heat sink temperature with lower compression ratio. Besides, the temperature glide in the generator and absorber of CAHP can be fitted to the heat source and heat sink, which achieves better coefficient of performance. Two models, respectively, for the generator and whole system using ammonia-water (NH3-water) as the working fluid are proposed, and the effects of different concentration of strong solution, cycle ratios, heat source temperature and spray density on the generator are investigated. The objective is to analyse the performance of CAHP system driven by low temperature geothermal water. The results show that the maximum of overall heat transfer coefficient of the vertical out-tube falling film generator can be obtained with the optimum spray density of around 0.16 kg/ms and there is an optimum concentration of around 65% for the CAHP system. When the low heat source temperature between 30℃ and 40℃, the high heat sink temperature can reach to 65-70℃.

Design and experimental study of a plate-type falling-film generator for a LiBr/H2O absorption heat pump

A novel plate-type falling-film generator for a LiBr/H2O absorption heat pump and the resulting generator/condenser unit prototype are presented in this paper. The generator consists of a directly-connected overflow solution distributor. This novel structure can ensure uniform solution distribution and stable falling-film flow; it is also more compact and provides greater feasibility to regulate capacity by plate assembly compared to conventional distributors for falling film. Visual flow pattern experiments were conducted in a vacuum atmosphere. Split flows and obvious parabolic dry areas were shown to exist. The method to improve the wetting rate was also investigated. Thermal performance tests show that the heat transfer coefficient of the generator ranged from 0.735 kW⋅m−2⋅K−1 to 0.856 kW⋅m−2⋅K−1. The flow and thermal experiment results demonstrate the feasibility of the plate-type falling-film generator for a LiBr/H2O absorption heat pump. The concept of the novel generator can be used for all absorption heat pump components and thus greatly increase the generalizability of the absorption technology.

Numerical simulation and experimental results of horizontal tube falling film generator working in a NH3–LiNO3 absorption refrigeration system

This paper describes the work made at the Centro de Investigacion en Energia in the development of an absorption refrigeration system for cooling and refrigeration applications with a capacity of 10 kW. The single effect unit utilizes ammonia-lithium nitrate as working pair and it is air cooled. The generator is a falling film type with horizontal tubes where the heating oil flows inside the tube bank and the ammonia-lithium nitrate solution flows as a falling film on the tube outside, where it is heated and ammonia vapor is generated. The generator consists of tree columns and four rows per column of horizontal tubes. The system was tested at controlled conditions with heating oil obtained from an electric resistance heating loop. A numerical model of the horizontal falling film generator was developed that divided the system into three different thermal elements: the flow inside the tube, the heat conduction in the tube wall and the falling film solution flow. The mathematical model was tested and validated with experimental data and a study of the influence of the heat transfer coefficient for ammonia-lithium nitrate solution in the numerical model was carried out. A comparison between experimental and numerical data for the heat flux in the system and the temperature profiles in the oil and solution flows shown a good degree of correlation.

Heat transfer performance of lithium bromide solution in falling film generator

An experimental investigation of vertical in-tube falling film heat transfer with different heat fluxes and concentrations of lithium bromide solution were conducted. The experiments show that the heat transfer coefficient increases with the decrease of inlet concentration and significantly increase with heat flux increase. An experimental correlation of falling film heat transfer coefficient is obtained: h m = 129.7712 W in - 0.8058 q w 0.2422 Re - 0.0856 The comparison of falling film generator with immersed tube generator shows that the heat transfer coefficient is 4.37 times higher than that of immersed tube generator, which can significantly reduce the volume of the falling film generator. The volume of falling film generator is only 52.1% of the volume of immersed tube generator.

Evaluation of absorption cycles with respect to COP and economics

The calculation of the performance of absorption heat pump cycles or the comparison of different types of machine cannot be done in a reasonable way without considering the first cost of the machine and especially the cost of the heat exchangers, as their area and particularly the distribution of the area between the respective components of the heat pump determine the COP. Therefore it makes sense only to compare machines that are optimized in this respect. A good way to evaluate cycles is to calculate the maximum COP in terms of the total cost of the heat exchangers. For that purpose a computer program was developed for different absorption heat pump cycles with water as refrigerant. The calculation method is simple and thus the result reliable. The program is suitable for evaluating double-lift, single-, double- and some triple-effect cycles, each one with different absorption fluids and with different options, such as different solution flows (parallel, serial), different types of absorber (spray- or falling-film absorber), and different types of generator (pool- or falling-film generator). With this instrument different cycles or similar cycles with different features can be compared. An economically significant estimation of the performance of a cycle working under defined conditions is possible.

Preparation of polymer colloids by chemical reactions in aerosols. I. Poly(p‐tertiarybutylstyrene)

AbstractA procedure was developed by which polymer colloids can be prepared by polymerizing aerosol droplets of monomers with an initiator in the vapor state. This work describes the formation of poly(p‐tertiarybutylstyrene) particles by exposing the corresponding monomer droplets to trifluoromethanesulfonic acid. It was found that the monomer‐to‐initiator mass ratio is the critical factor in determining the uniformity and morphology of the particles. Under ideal conditions spheres of narrow size distribution are obtained. Otherwise porous particles or soft particles with whiskers may develop. The aerosol was prepared in a falling film generator, but other procedures for producing monomer droplets may be used.

Preparation of uniform colloidal dispersions by chemical reactions in aerosols. I. Spherical particles of titanium dioxide

Spherical titanium dioxide particles were prepared by hydrolysis of aerosols consisting of liquid titanium(IV) compounds. The droplets of the latter were nucleated with AgCl in a falling film generator and subsequently hydrolyzed in specially designed chambers kept at different temperatures. When titanium(IV) ethoxide was used as starting material, TiO 2 spheres of narrow size distribution were obtained with modal diameters ranging between 0.06 and 0.6 μm. The size of the particles could be altered by changing the temperature of the falling film generator, the flow rate of the carrier gas, and the AgCl nuclei concentration. The titanium dioxide particles so prepared were essentially amorphous. Titanium tetrachloride was also used as the liquid aerosol material. Owing to the vapor pressure characteristics of this compound, much higher concentration of the droplets was obtained in the same equipment. However, the titanium dioxide powder, resulting from the hydrolysis of TiCl 4 aerosols, consisted of spheres of broader size distributions. The crystallinity of the solid particles could be changed by heating of the solid aerosol stream at different temperatures. The titanium dioxide powders, prepared by the aerosol technique, could be readily dispersed in water. The particle charge was pH dependent and showed an electrophoretic isoelectric point between pH 4.5 and 5.5.

Chemical reactions in aerosols. I. bromination of octadecene droplets

A monodispersed aerosol of 1-octadecene, produced in a falling film generator was reacted with bromine vapor at constant vapor pressure. The reaction time was controlled by changing the length of the glass tube through which the 1-octadecene aerosol and bromine vapor passed at a fixed flow rate. The particle size distribution of the aerosol was measured before and after bromination by light scattering and no significant changes were observed. The reaction products were collected using a thermal gradient precipitator and analyzed for the degree of bromination by infra-red spectroscopy. The factors that may control the kinetics of the aerosol-vapor reaction, such as gas or liquid phase diffusion, or the liquid phase reaction, are discussed. It is concluded that the liquid phase reaction between the 1-octadecene and the bromine dissolved in the droplet controls the rate of bromination under the conditions of these experiments.

Laboratory generation of aerosols

Laboratory methods for the preparation of aerosols are reviewed. Emphasis is placed upon techniques which are reproduceable and which result in a narrow size distribution so that the aerosols which are formed can be utilized in further laboratory investigations. Condensation generators have evolved from the Sinclair—La Mer generator. They are based upon controlled cooling of vapors and condensation nuclei in a flowing system. Although the design is usually quite empirical, a careful theoretical analysis has been carried out for the heat transfer and convective diffusion processes in the so-called falling film generator in which the evaporating liquid flows down the inner walls of a tube which is maintained at an elevated temperature. Aerosols comprised of solid particles have been prepared by similar techniques. These particles, which may vary over a very wide range of particle sizes, may consist of salts, metals, polymers or oil coated salts. Solid particles formed by quenching hot vapors are usually spherical and amorphous. A variety of metal oxide aerosols have been formed by burning metal chlorides and oxychlorides in specially designed burners. Dispersion methods include rotating disc and rotating cup sprayers, and vibrating orifice generators. These can yield highly monodisperse aerosols. The latter device is particularly advantageous since particle size can be calculated a priori from the operating conditions. On the other hand, compressed air nebulizers, ultrasonic nebulizers and electrical atomizers are more difficult to operate reproducibly and they give quite broad particle size distributions. Solid aerosols can be formed by any of the devices based upon a dispersion method by first forming an aerosol from either a solution of a non-volatile solute or a colloidal dispersion, and then evaporating the solvent. Dispersion of dry powders is not suitable for preparation of aerosols for laboratory investigations. Although gas phase reactions and photolysis are among the earliest techniques, they have not been utilized in recent years despite the advantages which they offer for formation of aerosols under well controlled conditions.

Brownian coagulation of aerosols at low Knudsen number

Dibutyl phthalate aerosols of narrow size distribution have been prepared in a falling-film generator using nitrogen rather than helium as the carrier gas. The Knudsen number in nitrogen is considerably lower so that the Cunningham correction is much less. A large number of coagulation experiments give average coagulation times in excellent agreement with theory. However, the spread of the values is much greater than obtained earlier in helium. The spread of the results reported here may be due to the lower thermal conductivity of nitrogen.