Martin Ryhl Kærn

Formulation and validation of a two-dimensional steady-state model of desiccant wheels

Desiccant wheels are rotary desiccant dehumidifiers used in air-conditioning and drying applications. The modeling of simultaneous heat and mass transfer in these components is crucial for estimating their performances, as well as for simulating and optimizing their implementation in complete systems. A steady-state two-dimensional model is formulated and implemented, aiming to obtain good accuracy and short computational times with the purpose of inclusion in complete system models. The model includes mass and energy balances and correlations for heat and mass transfer based on empirical relations from the scientific literature. Convective heat and mass transfer coefficients are computed locally accounting for the entrance length effects. Mass diffusion inside the desiccant material is neglected. Comparison with experimental data from the literature shows that the model reproduces the physical behavior of desiccant wheels satisfactorily, as the deviation between the computed results and available data is always within 12%. The simulation time is as low as 3 s for a model with 200 control volumes. It is found that for the applied case, the model provides accurate results for the overall flow using an equiangular control volume discretization with 200 control volumes and no axial discretization. More computationally expensive configurations with axial discretization give more accurate results and information on local flow and desiccant conditions inside the wheel.

Continuous versus pulsating flow boiling. Experimental comparison, visualization, and statistical analysis

This experimental study investigates an active method for flow boiling heat transfer enhancement by means of fluid flow pulsation. The hypothesis is that pulsations increase the flow boiling heat transfer by means of better bulk fluid mixing, increased wall wetting, and flow-regime destabilization. The fluid pulsations are introduced by a flow modulating expansion device and are compared with continuous flow by a stepper-motor expansion valve in terms of time-averaged heat transfer coefficient. The cycle time ranges from 1 to 9 s for the pulsations. The time-averaged heat transfer coefficients are reduced from transient measurements immediately downstream of the expansion valves at low vapor qualities. The results show that the pulsations improve the time-averaged heat transfer coefficient by 3.2% on average at low cycle time (1 to 2 s), whereas the pulsations may reduce the time-averaged heat transfer coefficient by as much as 8% at high heat flux (q ⩾ 35 kW/m2) and cycle time (8 s). The latter reduction is attributed to a significant dry-out that occurs when the flow modulating expansion valve is closed. Additionally, the effect of fluid flow pulsations is found to be statistically significant, disregarding the lowest heat flux measurements.