Analysis of multi-pass evaporators using orthogonal collocation

A thermal analysis of the evaporator of a water chiller system having a single shell and eight tube passes has been performed using the technique of orthogonal collocation. Refrigerant R-22 flows inside the tubes, and water flows in the shell. It is demonstrated here that the proposed approach is suitable for estimating the steady rate of heat exchanged and the overall pressure drop in the tubes, and presents itself as a suitable alternative to conventional numerical integration techniques. The predictions are made using well-known and contemporary two-phase heat transfer and pressure drop correlations. The use of a few collocation points is sufficient to predict results that are in good agreement with the results obtained with conventional integration. For the evaporator, the rated cooling effect is 140.67 kW and the rated overall pressure drop is 13.789 kPa. In comparison, the orthogonal collocation approach predicts values of 142.01 kW and 16.591 kPa, respectively.     

Particle size distribution of TBPB hydrates by focused beam reflectance measurement (FBRM) for secondary refrigeration application

The present work investigates TBPB (tetra-n-butylphosphonium bromide) hydrate slurries based on focused beam reflectance measurement (FBRM) method. Hydrates can be used as phase change materials (PCM) for thermal energy storage due to their high latent heat of melting. In this study, hydrates were formed from TBPB-water mixtures, suitable for air-conditioning applications. Moreover, hydrate slurries, composed of hydrate-particles dispersed in liquid, are appropriate solutions for cold distribution. Nevertheless, flow properties of hydrate slurries, depending on hydrate crystallization kinetics, must be controlled. In this work, FBRM was used for analysing TBPB-hydrate-particle size distribution. This work showed that FBRM can detect system phase changes (nucleation, growth, agglomeration). In addition, a bimodal distribution of particles was observed. This work could be useful to understand rheological behaviour of hydrate slurry, not only for secondary refrigeration applications, but also for natural gas hydrate transportation, pipeline plugging prevention, and gas capture.     

Axial dispersion in shell-and-tube heat exchangers

The effect of the deviation of the actual three-dimensional flow field on the shell-side from the frequently assumed one-dimensional uniform axial plug flow can be taken into account by superimposed axial dispersion in the fluid. The measure for axial dispersion is the Péclet number which can vary from infinite (no dispersion) to zero (complete axial mixing). For the fast and more reliable calculation of transient processes with the axial dispersion model, the Péclet number has to be known. A residence time distribution measurement technique for the determination of shell-side dispersive Péclet numbers is described and used to determine Péclet numbers for different shell-to-baffle clearances, numbers of baffles, and axial plug flow Reynolds numbers. Measurements with water reveal that Péclet numbers from 15 to 160 can occur and axial dispersion cannot be neglected in many cases.