Theoretical analysis of sudden expansion fittings in pneumatic conveying system

Theoretical analysis of minor loss coefficient for sudden expansions in pneumaticconveying system is the basic purpose of this thesis. The minor loss coefficient is anindication of the degree of permanent pressure or energy loss in the system. The flowstructure in pneumatic conveying systems characterized as two phase flow (gas-solid particle)is complex and difficult to investigate by experimental methods. Therefore, theoreticalanalysis of this flow regime is important.First, a literature survey on theoretical and experimental studies of sudden expansionpipe flows has been made. There are many studies on gas-liquid flows but scarcely on gassolidflows. Models on gas-liquid flows can be adapted to gas-solid flows when gaseous phaseis assumed incompressible. Experimental results are compared with theoretical models. Thedeficiencies in the theoretical analysis are defined. As a result, a method for the theoreticalanalysis of sudden expansion for gas-solid flows is presented.Then, homogeneous flow and separate flow approaches are utilized for gas-solidflows. By applying global laws (mass, momentum) in both approaches three differenttheoretical models are formed for the calculation of pressure difference arising in suddenexpansion pipe flows and also a theoretical model for gas-liquid flows available in literatureare developed for gas-solid flows. Slip flow models in literature is also shown for the relativevelocity between phases occurring during expansion.All the theoretical models are compared with experimental data of some authors inliterature. Results have shown that the separate flow model is the best among the existingmodels. The slip in velocity between phases is also considered and a hybrid model is formedby combining separate flow model with slip flow model and it has yielded very closeagreement with experimental data. As a result, the hybrid model can be considered as the bestsuitable theoretical model applicable to two phase gas-solid turbulent flow regimes.Finally, the pressure loss due to sudden expansion is divided as one due to gas alone andone due to particles so the pressure loss coefficient is formed as the sum of the pressure losscoefficient due to gas phase and additional pressure loss coefficient due to particles. It isresulted that addition of particles to the flow decreases the pressure loss along the expansionbut this decreasing in pressure loss is related with the material type to be conveyed or with therelative velocity existing between phases. It is also concluded that pressure loss coefficient intheory is not a function of Reynolds number in case of constant solid loading ratio with thesame particle.Keywords: sudden expansion, pressure loss, gas-solid flow, two-phase flow