Qiu Tu

Heating control strategy for variable refrigerant flow air conditioning system with multi-module outdoor units

The heating control strategy is key technology for stable and reliable heating operation of variable refrigerant flow air conditioning system with multi-module outdoor units. The synoptic control strategy is that the host module coordinates the system operation and adjusts the operation frequency of every module based on equal frequency allocation principle for suction pressure balance among modules. The heating control model including heating frequency adjustment, module switching condition and frequency allocation model has been built. Considering the discharge pressure disturbance, a certain discharge pressure region is taken as the control objective to adjust the frequency. And considering the lag between pressure and frequency, the restriction condition of discharge pressure and a certain judgment time are introduced into the model to avoid frequent module switching. Furthermore, different frequency allocation models have been built for increase and decrease of module number. In the model, when the module number increases and evenly allotted frequency to every module is less 70 Hz, one or two certain frequencies 70 Hz are distributed to the pre-existing thermo-on module(s) and the remaining frequency is distributed to ensuing thermo-on module so as to avoid sharp frequency fluctuation and inferior heating effect. Experiments demonstrate the feasibility of the control strategy.

CFD Simulation of Hydrodynamics of Three-Dimensional Circulating Fluidized Beds with EMMS Model

Unlike the conventional drag models which were developed on the basis of experiments with homogeneous system, energy minimization multi-scale (EMMS) drag model takes into account the heterogeneous structure inside the gas-solid flow. In this paper, the transient three-dimensional full-loop gas-solid flow in a pilot scale circulating fluidized bed (CFB) with six cyclones was simulated in the computational particle fluid dynamics (CPFD) method, based on EMMS. Particle volume fraction distribution, time averaged pressure profile, and averaged particle vertical velocity were analyzed and presented. The results show that under the simulation condition, in the dense region, particles agglomerate and form clusters; pressure drop evaluation of the dense region is still far away from the experimental data but better than using Wen-Yu model; vertical velocity profile shows core-annulus structure.