Numerical study on heat and mass transfer behavior of pool boiling in LiBr/H2O absorption chiller generator considering different tube surfaces

Investigating the pool boiling process in the absorption chiller generator by studying the valid parameters may enhance the chiller's coefficient of performance (COP). In the present study, the transient two-dimensional numerical modeling of LiBr/H2O solution pool boiling in the generator of the absorption chiller was carried out using the two-phase Eulerian- Eulerian approach, extended Rensselaer Polytechnic Institute (RPI) boiling model and renormalization group (RNG) K-? turbulence model. The numerical model was applied on three types of the bare, notched fin, and low fin tubes to investigate the effect of using fin on the boiling heat transfer rate in the generator of the absorption chiller and comparing it with the bare tube. Moreover, the numerical results were compared with the data obtained from the previous experimental studies to validate numerical modeling. A good agreement was achieved between numerical and experimental results. The results showed the evaporation mechanisms in the microlayer, evaporation in the three-phase (liquid-vapor-solid) contact line, and transient conduction to the superheat layer for constant thermal heat flux and the three surfaces of the copper tube within a specific period from the boiling point of LiBr/H2O solution. The results also showed that the use of a notched fin-tube and low fin tube increases the non-homogeneous nucleation rate, causes the solution to boil earlier than the bare tube, and reduces the required thermal energy in the generator of an absorption chiller.

An Experimental Investigation of the Air-Side Pressure Drop Through a Bare Tube Bundle

Presented in this paper is an experimental investigation of the steady-state air-side pressure drop through a delugable bare tube bundle. The tube bundle forms a part of a systematic investigation of the subcomponents of a hybrid dry/wet dephlegmator (HDWD), also known as a secondary steam condenser, used in direct dry-cooling. A modular bare tube bundle is designed and built to experimentally investigate the pressure drop in a counter-flow test facility with 19 mm and 25 mm tubes at various tube depths. The tests are conducted under both dry and wet (deluged) operating modes. The test results are used to develop empirical correlations for both dry and wet operating modes. The absolute average error is found to be 3.2% between the measured and predicted values for the dry correlation and for the wet correlation the absolute average error is found to be 4.5%. The newly developed correlation is used in a thermal case study to illustrate the effect of the under- and overprediction that the air-side pressure drop will have on a relatively small evaporative cooler where two cases corresponding to fan power consumptions of 1.1 kW and 5.5 kW were considered.