Siyoung Jeong

Dynamic simulation of an absorption heat pump for recovering low grade waste heat

Abstract Numerical simulation was carried out to predict the transient operating characteristics and performance of an absorption heat pump recovering waste heat of 30°C–40°C. The effects of the temperature and the mass flow rate of the heat transfer medium, the heat transfer area of the system components, and the solution circulation rate on the system performance are investigated in detail. The results obtained indicate that a higher heating capacity is obtained with an increase in driving steam temperature, waste heat temperature, and mass flow rate of hot water and waste water. It is also found that the heating capacity becomes higher with an increase in heat transfer area of the system components except solution heat exchanger. An increase in solution flow rate leads to higher heating capacity but to lower COP. A practical absorption heat pump, installed at a chemical plant, was also simulated numerically. The simulation results were compared with the operation data of the practical system.

Thermodynamic design data and performance evaluation of the water + lithium bromide + lithium iodide + lithium nitrate + lithium chloride system for absorption chiller

Abstract Duhring (P–T–X) and enthalpy-concentration (H–X–T) diagrams of the H2O + LiBr + LiNO3 + LiI + LiCl (mole ratio of LiBr : LiNO3 : LiI : LiCl = 5 : 1 : 1 : 2) system were constructed by using the experimental data sets. Thermodynamic design data for a double-effect series-flow absorption chiller were calculated at various operating conditions [ 2≤Te≤14°C, 30≤Ta≤50°C, 30≤Tc≤50°C, T gh COP =0 ≤T gh ≤T gh (crystallization limit)] by a computer simulation. The proposed working fluid was found to be applicable to cycle operation of air-cooled absorption chiller with no crystallization problem at higher absorber temperature.

Solubilities, Vapor Pressures, and Heat Capacities of the Water + Lithium Bromide + Lithium Nitrate + Lithium Iodide + Lithium Chloride System

The optimum mole ratio of lithium salts in the H2O + LiBr + LiNO3 + LiI + LiCl system was experimentally determined to be LiBr : LiNO3 : LiI : LiCl = 5 : 1 : 1 : 2. The solubilities were measured at temperatures from 252.02 to 336.75 K. Regression equations on the solubility data were obtained with a least-squares method. Average absolute deviations of the calculated values from the experimental data were 0.15% at temperatures <285.18 K and 0.05% at temperatures ≥285.18 K. The vapor pressures were measured at concentrations ranging from 50.0 to 70.0 mass% and at temperatures from 330.13 to 434.88 K. The experimental data were correlated with an Antoine-type equation, and the average absolute deviation of the calculated values from the experimental data was 2.25%. The heat capacities were measured at concentrations from 50.0 to 65.0 mass% and temperatures from 298.15 to 328.15 K. The average absolute deviation of the values calculated by the regression equation from the experimental data was 0.24%.

Optimal Design of Compact Horizontal Tube LiBr/Water Absorbers

A three-flow-regime (falling film, droplet formation, and droplet fall) model is used to investigate the possibility of using small diameter tubes for falling-film lithium bromide-water absorbers. A systematic investigation of tube diameters and pass arrangements shows that for the same heat transfer area, tubes with a diameter of 6.35 mm deliver about 30% more cooling capacity than conventional tubes (OD = 15.88 mm). An even higher increase in capacity (about 55%) is obtained when 3.175 mm diameter tubes are used. This significant increase is primarily due to increased vapor absorption in the droplet formation regime, which plays a progressively larger role in vapor absorption as the tube diameter decreases. However, because it is expected that wetting of tubes becomes poorer as the tube diameter decreases, wetting characteristics should be considered in conjunction with these findings to select the optimal tube diameter.

Evaporative Heat Transfer of CO2 in a Smooth and a Micro-Grooved Miniature Channel Tube

Grooved surfaces are widely used to enhance heat transfer. In this study, micro-grooves are applied to miniature channels to improve the evaporative heat transfer of CO2. To evaluate the effect of micro-grooves in miniature channels, heat transfer characteristics are investigated for the smooth multichannel tube with 0.8 mm channel diameter and the grooved multi-channel tube with the same diameter and 8 micro-grooves. The tests were carried out at the evaporation temperatures of 0, 5 and 10°C; mass flux between 400 and 800 kg/m2s; and heat flux of 12–18 kW/m2. At lower qualities (less than about 0.3), heat transfer was considerably enhanced by the grooves, however, the micro-grooves showed negative effect in heat transfer at higher qualities. A significant heat transfer enhancement was obtained at high evaporating temperature. Also, heat transfer was enhanced at lower heat and mass flux. The evaporative heat transfer coefficients measured were compared with several correlations. A new correlation was developed to predict the dry-out quality in miniature channel tubes more accurately.

Evaporative Heat Transfer and Pressure Drop of CO2 in a Microchannel Tube

Evaporation heat transfer and pressure drop characteristics of carbon dioxide were investigated in a multi-channel micro tube. The aluminum tube has 3 square channels with a hydraulic diameter of 2mm, a wall thickness of 1.5mm, and a length of 5m. The tube was heated directly by electric current. Experiments were conducted at heat fluxes ranging 4–16 kW/m2 , mass fluxes from 150 to 750 kg/m2 s, evaporative temperature from 0 to 10°C, and qualities from 0 to superheated state. The heat transfer coefficient measured was in the range of 6–15kW/m2 K, and the pressure drop was 3–23kPa/m. For the qualities lower than 0.5, the heat transfer coefficient was found to increase with the quality, which is assumed to be the effect of convective boiling. For the qualities higher than 0.6, sudden drop in heat transfer coefficients was sometimes observed due to local dry-out. It was found that dry-out occurred at lower quality if mass flux was smaller. The average heat transfer coefficient was found to increase with increasing heat flux, mass flux, and evaporation temperature, of which the effect of heat flux was the greatest. At given experimental conditions the pressure drop increased almost linearly with increasing quality. The total pressure drop was found to increase with increasing heat flux, mass flux, and evaporation temperature, of which the effect of mass flux was the greatest. From the experimental results simple correlations for heat transfer coefficients and pressure drop were developed.© 2005 ASME

Pumping characteristics of a thermosyphon applied for absorption refrigerators with working pair of LiBr/water

Abstract A thermosyphon with a separate heating chamber was taken into consideration as a model device for a high-temperature generator in a small capacity absorption refrigerator with LiBr/H 2 O solution. Quantitative measurements of operating characteristics of the thermosyphon were made as functions of pipe diameter, pumping height and heating power. It was found that vapor production rate is mainly affected by the heating power, and the flow rate of pumped liquid is limited by the pumping height. It was also observed that, as the pipe diameter increases, dependency of liquid flow rate on the pumping height becomes stronger. For a LiBr/H 2 O solution system, ratio of vapor production rate to liquid flow rate was found to be well correlated as a function of the power input. Present experimental data are expected to provide useful information for determination of design parameters of a thermosyphon.

Numerical Simulation of a Two-Stage Hybrid Heat Pump

초록: 기존의증기압축식과흡수식을결합한하이브리드히트펌프는50 정도의저열원에서80~90 의온수를효과적으로생산할수있다. 본연구에서는EES를사용하여2단압축하이브리드히트펌프와1단압축하이브리드히트펌프의성능을비교하였다. 동일한작동조건에서2단압축하이브리드히트펌프는1단압축하이브리드히트펌프보다약간높은COP를가지며더안정적인상태에서운전이가능한것으로나타났다. 2단압축하이브리드히트펌프에서작동유체의최대온도는1단압축하이브리드히트펌프보다40K정도낮게나타났으며이는윤활유의작동에무리없는운전상태를가능하게한다. 1단과2단하이브리드히트펌프모두UA값이증가할때COP는감소하였으며열출력은증가하였다.Abstract: Hybrid heat pumps, which combine the vapor compression and absorption heat pump cycle, can efficientlyproduce hot water of 80°–90°C from the low temperature of ~50°C. In this study, the performance of a two-stagehybrid heat pump (HHP) was compared with a single-stage hybrid heat pump using EES (Engineering EquationSolver). For the same operating conditions, the two-stage HHP showed a slightly higher COP (Coefficient OfPerformance) and more stable operating conditions than the single-stage HHP. Moreover, the maximum working fluidtemperature of the two-stage HHP was found to be lower than that of the single-stage HHP by about 40 K, whichmakes the working conditions of the lubricating oil safer. The COPs of both systems decreased with increasingUA-values. However, the heat output of the HHP was increased at the same time.

Heat Transfer Characteristics During Gas Cooling Process of Carbon Dioxide in a Horizontal Tube

The heat transfer coefficient and pressure drop during gas cooling process of carbon dioxide in a horizontal tube were investigated. The experiments were conducted without oil in the refrigerant loop. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flowmeter, an evaporator, and a gas cooler(test section). The main components of the water loop consist of a variable-speed pump, an isothermal tank, and a flowmeter. The gas cooler is a counterflow heat exchanger with refrigerant flowing in the inner tube and water flowing in the annulus. The test section consists of smooth, horizontal stainless steel tube of the outer diameter of 9.53mm and of the inner diameter of 7.75mm. The length of the test section is 6m. The refrigerant mass fluxes were 200∼300kg/(m2s) and the inlet pressure of the gas cooler varied from 7.5㎫ to 8.5㎫. The main results were summarized as follows : Pressure drop of CO2 increases with increasing gas cooler pressure. The friction factors of CO2 in a horizontal tube show a relatively good agreement with the correlation by Blasius. The heat transfer coefficient of CO2 in transcritical region increases with decreasing gas cooler pressure and decreasing mass flux of CO2. Most of correlations proposed in a transcritical region showed significant deviations with experimental data except for those predicted by Gnielinski.

Two-Phase Flow Regimes for Counter-Current Air-Water Flows in Narrow Rectangular Channels

A study of counter-current two-phase flow in narrow rectangular channels has been performed. Two-phase flow regimes were experimentally investigated in a 760 mm long and 100 mm wide test section with 2.0 and 5.0 mm gap widths. The resulting flow regime maps were compared with the existing transition criteria. The experimental data and the transition criteria of the models showed relatively good agreement. However, the discrepancies between the experimental data and the model predictions of the flow regime transition became pronounced as the gap width increased. As the gap width increased the transition gas superficial velocities increased. The critical void fraction for the bubbly-to-slug transition was observed to be about 0.25. The two-phase distribution parameter for the slug flow was larger for the narrower channel. The uncertainties in the distribution parameter could lead to a disagreement in slug-to-churn transition between the experimental findings and the transition criteria. For the transition from churn to annular flow the effect of liquid superficial velocity was found to be insignificant.