Showing papers on "Laminar flow reactor published in 1990"

On Laminar Flamelet Modelling of the Mean Reaction Rate in a Premixed Turbulent Flame

Abstract: The modelling of the mean reaction rate in a premixed turbulent flame is approached by means of a library of laminar flame solutions. A formulation of such a library is presented and the detailed data requirements are investigated. Laminar flame calculations are performed in an asymmetrical counterflow configuration with a cold, reactive methane-air mixture flowing against the corresponding hot equilibrium combustion products. The flamelet properties required for the library are shown to be independent of the definition of a reaction progress variable c. Stoichiometric flames at pressures of 1 bar, 1.5 bar and 2 bar are considered, with corresponding initial temperatures of 328, 386 and 433 K. The results are examined in the light of the demands of the library, and suitable definitions of key quantities are proposed. A specimen laminar flamelet library is then assembled.

METHODS to DISTINGUISH BETWEEN LAMINAR and VORTICAL FLOW IN SCRAPED SURFACE HEAT EXCHANGERS

Abstract: The transition between laminar and vortical flow has been investigated in scraped surface heat exchangers (SSHEs), using new methods based on viscosity and temperature measurements. The transition occurred close to the critical Reynolds number for annular flow of Newtonian fluids. Thus, the methods used to predict the viscosity can be successfully applied to SSHEs despite the very complex flow behaviour of the products, the complex geometry of the SSHEs, and the heat transfer in the SSHEs. In industrial applications, the transition can be detected using the new method based on temperature measurements at the outlet of the SSHEs together with some calculations. This is a very important result, since with this “sensor” the flow pattern can be controlled towards the optimal flow pattern in SSHEs, which in turn is a fundamental requirement when the taste of aseptic products containing particulates are to be improved by the introduction of continuous sterilization.

Divergent Flow in Chemical Vapor Deposition Reactors

Abstract: An undesirable flow phenomenon can arise from divergent flow at the entrance region of a chemical vapor deposition (CVD) reactor, leading to uneven and recirculating backflow and correspondingly poor growth uniformity downstream. This phenomenon, recognized only recently in the CVD field, is caused by flow separation at the reactor wall. This type of flow is known as Hamel or Jeffery‐Hamel flow for two‐dimensional planar diverging channels. The onset of Hamel flow depends on gas Reynolds number, reactor divergence angle, and for channel‐type reactors, reactor aspect ratio. The transition from stable flow to Hamel flow was studied experimentally and compared with analytic predictions for idealized geometries. Idealized analytic predictions are shown to be useful for qualitatively interpreting flow behavior, but they are poor guides for quantitative design of CVD reactors of complex entrance geometry. Experimental determination of the maximum allowable divergence angle for a specific flow rate and reactor configuration is required.

Heat transfer solutions in laminar co-current flow of immiscible liquids

Abstract: Thermal entry region solutions are analytically determined for horizontal, co-current laminar flow of immiscible liquids in direct contact, inside circular tubes and parallel plate channels. The related eigenvalue problem for such a composite media is readily solved by extending the ideas in the recently advanced sign-count method. It is assumed a core-annular flow configuration for circular tubes and sheat-core flow for the parallel plates channel, without consideration of interface instabilities and stratified flow. First, the velocity problem is solved for fully developed flow and pumping power expressions established for different operating conditions. Then, the temperature problem is analytically handled to yield expressions for quantities of practical interest such as total heat exchange rates, along the duct length and, again, for different flow rates and pressure drop requirements. The analysis is illustrated through consideration of an application dealing with pumping of a very viscous oil with the addition of an external thin layer of a less viscous fluid (water). Pumping power and total heat exchange are then evaluated for both geometries and critically compared to the single fluid flow problem.

Sedimentation of Charged Fibers From a Two-Dimensional Channel Flow

Abstract: An analysis is presented to describe the motion of a charge-carrying fiber under gravity in a laminar flow, between two horizontal conducting parallel plates. Using the method of limiting trajectories, the precipitation efficiency of a uniform aerosol that enters the channel is determined for both insulating and conducting fibers.

Radiative interactions in laminar incompressible and compressible internal flows

Abstract: Analyses and numerical procedures are presented to investigate the radiative interactions of gray and nongray absorbing-emitting species between two parallel plates and in a circular tube. Laminar fully developed incompressible as well as entrance region subsonic flows are considered. The participating species considered are OH, CO, CO2, CH4, and H2O. Results obtained for different flow conditions indicate that the radiative interactions can be quite significant in fully developed incompressible flows. For subsonic flows, however, the flowfield is not changed significantly due to radiative interactions.

Topics in vacuum system gas flow applications

Abstract: Evacuation to high vacuum utilizes gas flow in the choked, laminar, transition, and free molecular flow ranges. Understanding the gas flow equations applicable in these different ranges frequently explains the physical phenomena which are occurring as evidenced by the following situations. Contaminants are removed from systems and mechanical‐pump oil backstreaming is eliminated by using forced gas flow operation in the laminar viscous flow range. The mean free path dependency on pressure explains the independent behavior of condensible gases flowing from the system to a liquid nitrogen trap while pump‐generated permanent gases are flowing backward through the trap to the system. The highly directional gas flow which occurs in space simulation chambers can be combined with good high‐vacuum technology to obtain improved processing for applications such as molecular beam epitaxy. The current understanding of gas flow in capillaries demonstrates that the large pressure drop where the leak exits into a high‐va...

Superfluid turbulence in non-uniform thermal counterflow

Abstract: We report preliminary results from a study of superfluid turbulence in a non-uniform thermal counterflow. These results are important for understanding how a vortex tangle is distributed in a spacially varying velocity field, and for describing the radial transport of heat in He II. Using a specially designed flow tube and a very high sensitivity thermocouple we are able to measure the temperature difference associated with thermal counterflow in a known non-uniform flow. We observe laminar flow, a critical heat current, and a single state of turbulent flow. The results indicate that the vortex line density in the supefluid turbulence is determined simply by the local velocity.

格子乱流中の拡散火炎におけるWrinkled Laminar Flameletモデルの検討

Abstract: The temperature has been measured in a field in which laminar hydrogen (6.0 m/s at the center of a burner exit) was injected into a co-flowing air stream (6.0 m/s) with some kinds of grid turbulence. It is shown that the ratio of a laminar flame length to the turbulent one is proportional to the turbulent Reynolds number, Reι, (integral scale is considered a characteristic length) in the measured extent (Reι≤1000), and therefore that the integral scale of a reactant flow is an important parameter for the flames in this paper. The width of a flame displacement is estimated from the temperature profiles when the flame is assumed to be a wrinkled laminar flamelet. The width is also measured directly by a flow visualization method. Both the widths obtained by the two different methods agree well with each other. This fact indicates that the turbulent flame in this paper can be treated as a wrinkled laminar flamelet.

Rotational instability of the laminar flow of pseudoplastic fluids in a coaxial-cylindrical channel

Abstract: Measurements of the stability limit of a spiral flow of a non-Newtonian fluid (n ≤1) in a coaxial-cylindrical channel are presented and substantiated.