Forced Air Flow Research Articles

The Use of Unheated Air in the Drying of Wool

In drying wool by forced air flow to a uniform regain in the range 8% to 16%, the amount of water removed per unit volume of air used is almost independent of the air temperature. , In view of this, the paper examines the practicability and economics of using unheated air for wool drying. Running costs depend on the atmospheric humidity , at the location of the dryer, but usually are appreciably lower than those of an efficient steam-heated dryer. The air intake per pound of water removed is much greater than in conventional dryers but the blower power required is approximately the same. No circulation is used, and this permits close control of the regain of the wool leaving the dryer.

The Drying of a Mass of Hygroscopic Fibers by Forced Convection of Air: A Note on Some Physical Aspects

It is pointed out that, in drying of a uniform mass of fibers by forced convective air flow through the mass, a "drying front" proceeds through the mass in the same direction as, but much more slowly than, the air flow. Therefore, if uniform drying is to be obtained, the process must continue until the front has emerged from the downstream face of the mass of fibers; air of appropriate relative humidity must be used to leave the fibers at the desired regain; and reversal of air flow direction is undesirable on grounds of efficiency. Calculations have been made of the capacity of air to remove water from a mass of wool fibers, the mass being left at given values of regain (4, 8, 12, and 16%). It is found that, except for the case of 4% regain, the capacity for water removal depends very little on the initial temperature of the air (in the range 30°-110° C.).

Convective Heat Transfer for Mixed, Free, and Forced Flow Through Tubes

Abstract This paper presents current knowledge on heat transfer caused by simultaneously occurring free and forced convection in tubes. It discusses the results of experiments that have been conducted by the NACA on mixed, free, and forced-convection heat transfer in turbulent flow through a vertical tube with a length-to-diameter ratio of 5. In these studies, forced air flow was directed parallel or opposite to the flow direction which free-convection flow alone would have under the same temperature conditions. Results of experiments made by other investigators, when analyzed in the same way as the NACA experiments, make it possible to generalize and extend the heat-transfer relations established. A calculation by R. C. Martinelli and L. M. K. Boelter for laminar mixed, free, and forced flow in parallel direction, and its extension to flow in opposite direction furnish information on heat transfer in the laminar-flow range.

Effect of Air Movement on Combustion Rates in Spark-Ignition Engines

A movement within the combustion chamber of an internal-combustion engine can be divided into two general classifications: (1) air swirl and (2) air turbulence. Air swirl is defined as a regular rotation, as a whole, of the charge within the cylinder or the combustion chamber. Air turbulence is defined as irregular movement of the charge within the cylinder or the combustion chamber; turbulence may be either small scale or large scale. The air movement may be produced during the induction period, in which case it is called * 'induced'' air movement; or it may be produced by forcing the charge through a restricting passage within the cylinder or the combustion chamber during the compression stroke, in which case it is called ''forced air movement. In spark-ignition engines for aircraft, the necessity of a simple combustion-chamber shape to provide high volumetric efficiencies and adequate cooling has resulted in little attention being paid to forced airflow. Since, however, the air or the charge enters the cylinder at high velocities, a certain amount of induced air movement is always produced in the cylinder and the combustion chamber. This movement, once started, persists to a varying degree during the entire compression stroke. Air movement within the engine cylinder has several different effects on the combustion of the charge. It may effect the homogeneity of the mixture, the rate of heat transfer from the mixture to the combustion chamber walls, and the rate of combustion when the movement persists through the combustion period. The first two effects may, in themselves, affect the combustion rate. It is the purpose of this paper to present data obtained at the Langley Memorial Aeronautical Laboratory illustrating the two different classifications of air movement and to present data that show some of the effects of these movements on the rates and regularity of combustion.

Abridgment of surface heat transfer in electric machines with forced air flow

Since the insulation of windings in electric machines has comparatively low temperature limits, the problem of cooling these machines with the most economical use of material becomes one of major importance. The design of such machines from a temperature standpoint is usually based on tests of a previously made similar machine or else is of the “cut-and-try” type where such tests are not available. The predetermination of the operating temperature depends a great deal upon the rate at which the heat losses can be liberated from the ventilating surface to some cooling fluid such as air, which is considered in this paper. Some data are available regarding this rate of heat dissipation with forced air convection currents; a comparison of the various results published, however, shows them to be inconsistent. The purpose of this paper is to submit additional information that should be of value to the industry and that will also explain some of the inconsistencies in the past tests.

Surface Heat Transfer in Electric Machines with Forced Air Flow

Since the insulation of windings in electric machines has comparatively low temperature limits, the problem of cooling these machines with the most economical use of material becomes one of major importance. The design of such machines from a temperature standpoint is usually based on tests of a previously made similar machine or else is of the ``cut arnd try'' type where such tests are not available. The predetermination of the operating temperature depends a great deal upon the rate at which the heat losses can be liberated from the ventilating surface to some cooling fluid such as air, which is considered in this paper. Some data are available regarding this rate of heat dissipation with forced air convection currents; a comparison of the various results published, however, shows them to be inconsistent. The purpose of this paper is to submit additional information that should be of value to the industry and that will also explain some of the inconsistencies in the past tests. Extensive experimental tests were made regarding the rate of surface heat transfer with air flows of various velocities for radial and axial ducts. The influence of shape, size, cross-section, condition of surface, mean temperature, and other factors were investigated. The most important factor found was that this rate of heat transfer was not constant along the air flow path but that its value varied from point to point along the duct.