Impinging-Stream Systems

Impinging streams [73] is a unique and multipurpose configuration of a two-phase suspension for intensifying heat and mass transfer processes in the following heterogeneous systems gas-solid, gas-liquid, liquid-liquid and solid-liquid. 

The intensification of the transfer processes is due to the following effects a) An increase of the relative velocity between the penetrating particles and the opposed gas stream. Under extreme conditions, where the particle attains the gas velocity at the point of entering the opposite stream, the relative velocity may 

In section 4.5, the following designations are made Qi, Qij and qij are the mass flow rates of the particles stream (kg particles/sec) and Cj is the concentration of the tracer particles (kg tracer particles/kg particles). In such systems, the tracer particles are, usually, those of the original ones. They are, however, made radioactive or are painted, in order to distinguish them from the original particles. The latter makes it possible to determine their concentration versus time in the RTD experiments [73, p.l76], thus their mean residence time tm in the system. tm = 

Vi/Qi in a perfectly mixed reactor. An additional key quantity is the holdup of the particles in the reactor, Vi (kg particles) instead of the volume of the reactor. Thus, in consistent units, Eqs.(4-1) to (4-11) hold. An important quantity used in this section for comparing various effects is the mean residence time tm of the particles in the system. 

The simplest model [73, p.l80] of a two impinging-stream reactor is shown schematically in Fig.4.S-l. On the LHS is demonstrated the actual configuration of the reactor and on the RHS the Markov-chain model. The latter employs the following considerations and assumptions  

---

The main significance of the terminal velocity lies in the fact that it defines the minimum gas velocity requested. The particle enters the system at Point 1 at zero velocity. If a gas velocity is employed so that u, < uh the particle cannot get into but drops out of the system if the gas velocity makes the relative velocity equal the terminal velocity, i.e., u, = it, the particle will move with a constant velocity of zero with respect to the ground, and so will remain at Point 1 while if the gas velocity makes the relative velocity greater than the terminal velocity, i.e., u,> it, the particle will be accelerated by the gas flow to the terminal velocity and will then move towards Point 2 at that velocity. Therefore, the terminal velocity is necessary for the determination of the operational range of the gas flow velocity for a specific vertical gas-solid impinging stream system. 

Mean residence time tm of the impinging-stream systems in Fig 4.5-5... 

For any impinging stream system, the heat transfer coefficient, both local and averaged over the total residence time of particles, increases as the gas velocity increases and decreases as the gas temperature increases. Such a temperature influence on heat transfer results not only from the variation in gas properties but also from an appreciable change in the hydrodynamic conditions with an increase in the gas temperature, the penetration depth into opposing gas stream increases, which extends the period of unsteady particle motion, thus enhancing the total heat transfer. 

Impingement demister systems are designed to intercept liquid particles before the gas outlet. They are usually constructed from wire mesh or metal plates and liquid droplets impinge on the internal surfaces of the mist mats or plate labyrinth as the gas weaves through the system. The intercepted droplets coalesce and move downward under gravity into the liquid phase. The plate type devices or vane packs are used where the inlet stream is dirty as they are much less vulnerable to clogging than the mist mat. 

Example 5-8 Turbine Blade. Consider a fluid stream impinging on a turbine blade that is moving with a velocity Vs. We would like to know what the velocity of the impinging stream should be in order to transfer the maximum amount of energy to the blade. The system is the fluid in contact with the blade, which is moving at velocity Vs. The impinging stream velocity is V, and the stream leaves the blade at velocity Va. Since V0 = Vro + Vs and V = Vri + Vs, the system velocity cancels out of the momentum equation ... 

In the impinging streams of gas-liquid systems, high relative velocity between phases and collision between droplets favor surface renewing of droplets, resulting in reduced liquid film resistance and thus increased overall mass transfer coefficient. 

Figure 1 represents the basic principles of gas-solid impinging streams, and also its essential structure as originally designed. On the basis of the essential structure, various devices can be constructed by extending the idea of impinging streams. Two extension schemes of IS have been proposed extension of the flow configuration and extension of the phase conditions of the substance systems involved, as described below. 

In addition, the method of impinging streams can also be used for systems of single phase, such as gas-gas and liquid-liquid impinging streams etc. In fact, single phase impinging streams have great value for practical application in mixing, gas combustion, etc. 

The idea of impinging streams (IS) was originally presented for enhancing transfer processes in gas-solid systems. In the 30 years from 1961 when the concept of IS was presented by Elperin to the mid-1990s, investigations on IS were mainly concentrated on systems with a gas as the continuous phase, while, to an extent, the dispersed phase was extended to include liquid. 

As mentioned before, the concept of impinging streams (IS) was originally suggested for enhancing heat and mass transfer between a solid and a gas, and the development of the application of impinging streams were long focused mostly on multiphase systems. However, the problems involved in multiphase impinging streams are considerably complex so that it is necessary to study the behaviors of the individual phases separately. 

Consider the equipment system schemed in Fig. 3.8, where the subscript B denotes the process particles and A the tracer. Corresponding to the impinging stream equipment with feeding on both sides, the system has two feeds, which are denoted by the superscripts (1) and (2), respectively while it has only one out stream of particles. For convenience of operation, the tracer A is inputted into one feeding stream, i.e., (2) ... 

On the resistance constitution of the equipment system, the major conclusions that can be drawn from the investigation are (1) Where millets or rapeseeds are the material to be processed, the power for the operation of the impinging stream contactor is mainly (>80%) consumed in the acceleration of particles (2) The pressure loss due to the impingement between the opposing streams is independent of the presence of solid particles. 

Gas-continuous impinging streams with a liquid as the dispersed phase has wide application, such as in the combustion of liquid fuel droplets, absorption, water-spray cooling of air, etc. [9]. In such systems the dispersity of liquids plays a very important role affecting heat and mass transfer rates, because it influences both the interface area and the mean transfer coefficient. Wu et al. [68] investigated the influence of impinging streams on the dispersity of liquid. 

Figure 6.1 Impinging stream drying system for aluminum sulfate [86, 87].

---