Confined jets

While this parameter varies between unity and infinity, experience has shown that for short and intense flames encountered in cement kilns, the recirculation is such that the Craya-Curtet parameter M 2.0. Flames with M between 1 and 2 are characterized as long flames with the intensity suitable for processes such as rotary limestone calcination kilns. For flames with M 1.0, the entrainment is such that the flame tends to be long and lazy. The Craya-Curtet parameter. 

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Most of the remarks above refer to unconfined or free flows. Many industrial appHcations involve the use of confined jets. It is customary to consider a jet confined when the ratio of the confinement radius to the source radius Hes in the range 4—100. Below a ratio of 2, the jet does not develop its similarity profile before striking the wall, whereas above a ratio of 100 the jet itself may usually be considered free. Under certain conditions, flow in confined jets is accompanied by the existence of a recirculation 2one which significantly affects the jet behavior by returning material upstream (9). This recirculation can be particularly important in combustion processes. 

Characteristics of the air jet in the room might be influenced by reverse flows, created by the jet entraining the ambient air. This air jet is called a confined jet. If the temperature of the supplied air is equal to the temperature of the ambient room air, the jet is an isothermal jet. A jet with an initial temperature different from the temperature of the ambient air is called a nonisother-mal jet. The air temperature differential between supplied and ambient room air generates buoyancy forces in the jet, affecting the trajectory of the jet, the location at which the jet attaches and separates from the ceiling/floor, and the throw of the jet. The significance of these effects depends on the relative strength of the thermal buoyancy and inertial forces (characterized by the Archimedes number). 

Abramovich was the first to study axisymmetric confined jets analytically. He suggested the method based on utilizing the equations of continuity and momentum conservation. He also assumed that the width of the layer of a jet mixing with a counterflow equals the width of a free jet with a velocity distribution according to Schlichting s formula ... 

TABLE 7.17 Results of Experimental and Analytical Studies of the Compact Confined Jet... 

Experimental studies conducted by Grimitlyn on heated and chilled confined jets showed that the airflow pattern remains the same as for isothermal... 

E. (2007) Study of scalar macro- and microstmctures in a confined jet. 5th International Symposium on Turbulence and Shear Flow Phenomena,... 

Verge, 1995] Verge, M. P. (1995). Aeroacoustics of Confined Jets with Applications to the Physical Modeling of Recorder-Like Instruments. PhD thesis, Eindhoven University. 

Borner, Th., Durst, F. and Manero, E., LDV measurements of gas-particle confined jet flow and digital data processing, Proc. 3rd Int. Symp. on Applications of Laser Anemometry to Fluid Mechanics, Paper 4.5. (1986)... 

Confined Jet with Sufficient Amount of Surrounding Flow. 

The confined jets have been thoroughly studied by Thring and Newby[5], as well as at the International Flame Research Foundation (IFRF) at IJmuiden-Holland. Thring and Newby proposed a simple theoretical treatment of the problem based on the assumption that the rate of the entrainment of the jet is unaffected by the enclosure, but the spread out of the jet is a function of its momentum flux. 

In all other cases, the existence of external recirculation will depend on the geometry of the enclosure and the supply of surrounding flow. In the case of a sufficient amount of surrounding flow, the confined jet behaves as a free jet. 

If that is not the case then for an isothermal confined jet the Thring-Newby [5] parameter acts as a similitude criteria to scale up from model to prototype ... 

The surrounding flow is less than the amount that can be entrained by the confined jet. The following assumptions are taken into considerations The confined jet expands as a free jet with angle a = 19.4° and thus impinges on a wall at the distance Xp. The confined jet behaves as a free jet up to point C, at abscissa Xc (i.e., entrainment ceases and disentrainment begins) downstream, midway between the point where entrainment is zero is the point N with abscissa % and the point P where the jet impinges the wall has the abscissa Xp (see Figure 31.9). 

Craya and Curtet [4] have used averaged integrated values of Navier-Stokes equation as well as the continuity equation to predict the confined jet behavior. The dimensionless Craya-Curtet parameter m is given by ... 

Main Jet Similitude Parameters for Simple Confined Jets... 

Some recirculation is essential for flame stability. In many cases, the external recirculation originating from confined jets is insufficient to maintain a constant supply of hot gases and/or active species to maintain ignition. In this case, recirculation of hot gas must occur by some other means. The presence of swirl or of tangential velocity component results in the setup of radial and axial pressure gradients. The radial pressure gradient, given by... 

Curtet, R. "Confined Jets and Recirculation Phenomena with Cold Air." Combustion and Flame 2, no. 4 (1959) 383-411. 

Barchilion, M., and Curtet, R. Some Details of the Structure of an Axisymmetric Confined Jet with Backflow, Journal Basic Engineering (Transaction ASME), 86 (1964) 777-87. 

Fire Type Open jet fire Confined jet fire Open pool fire Large or confined pool fire... 

The sensitivity of random input parameters (radiative and convective heat fluxes, flame emissivity and convection heat transfer coefficient) to the output variable of the model (time-to-loss-of-strength) have been performed in three case studies for confined jet fires and the ranges of variables in Table 2. Case four has been carried out to cover the full range of flame emissivity in both Table 1 and 2. The data used in these studies are summarised in Table 4. 

The uncertainty intervals of these four input random parameters are for the confined jet fire in Table 2. No additional information about the probabUity distributions of input random variables and their mutual relationships is assumed for modelling. For this reason, the randomness of input parameters was modelled by the independent uniform distributions between the lower and upper values in the range. Due to the robustness ofthe simulation approach, it is possible to extend the presented approach and to include more complicated behaviour of the input parameters, when this information becomes available (Ref. 9). The simulation model was implemented in MS Excel and run 100 times by the Monte Carlo method directly in the MS Excel environment. 

Craya-Curtet number (dimensionless ratio of thrusts and jet momentum controlUng flow in confined jets where the primary jet is much smaller than the dimensions of the confining enclosure)... 

Curtet R. (1958) Confined jets and recirculation phenomena with cold air. Combust. Flame,... 

Hedges KR, Hill PG. (1972) A Finite-difference method for confined jet mixing. Thermal and Fluid Sciences Group, Department of Mechanical Engineering, Queen s University, Kingston, Ontario, Canada. 

This chapter presents the rotary kiln freeboard aerodynamic phenomena, drawing parallels with fluid flow in conduits. The goal is to describe the characteristics of confined jets that determine burner aerodynamic mixing and, in turn, combustion efficiency, and flame shape and its character. Having described the flow field, the effect of turbulence on dust pick-up from the bed s free surface will also be discussed. 

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