Properties of AB Materials PDF

PROPERTIES OF AB MATERIALS AERO DYNAMIC PROPERTY LECTURE 4:00 -6:00 PM LERJUN M. PENAFLOR PHD IN AGRICULTURAL ENGINEERING AERO OR HYDRO DYNAMIC PROPERTIES Aero and hydro dynamic properties are very important factor in hydraulic transport and handling as well as hydraulic sorting of agricultural...

PROPERTIES OF AB MATERIALS AERO DYNAMIC PROPERTY LECTURE 4:00 -6:00 PM LERJUN M. PENAFLOR PHD IN AGRICULTURAL ENGINEERING AERO OR HYDRO DYNAMIC PROPERTIES Aero and hydro dynamic properties are very important factor in hydraulic transport and handling as well as hydraulic sorting of agricultural product. To provide basic data for the development of equipment for sorting and sizing of agro commodities, several properties such as: physical characteristics and terminal velocity are needed. The main properties which affect the aerodynamic behavior of agricultural products are the aerodynamic drag / drag coefficient and the terminal velocity. By defining the terminal velocity of different threshed materials, it is possible to determine and set the maximum possible air velocity in which material out of grain can be removed without loss of grain or the principle can be applied to classify grain into different size groups. In addition, agricultural materials and food products are routinely conveyed using air. For such operations, the interaction between the solid particles and the moving fluids determine the forces applied to the particles.  FLUIDS v A fluid is a substance which is amenable to distortion when a force is applied upon it. v Vapours, gases and liquids are classified as fluids. v They can flow through pipes, conduits and open channels with the help of differential pressure. v In foods hydrodynamics mainly deal with water which is an incompressible fluid. v Viscosity, pressure and density are the major characteristics of liquids which influence its dynamic properties. v Fluid flow occurs around agricultural products often in transport or for separating the desired product from the unwanted material. In this case, the problem involves the action of the forces exerted by the fluids on these products. DRAG FORCE Opposes the motion of an object. the drag force is proportional to some function of the velocity of the object in that fluid. depends upon the shape of the object, its size, its velocity, and the fluid it is in. Drag force (FD) is proportional to the square of the speed of the onal to the square of the speed of tobject. We can write this relationship mathematically as:  DRAG COEFFICIENT Consider the object shown as fluid flows when it is immersed, the action of the forces involved include Fr the force acting on the body which is resolved in to components FD drag and FL the lift. These forces depend on the area Ap moving through, fluid density ρf , viscosity η and velocity V.  DRAG AND LIFT COEFFICIENT The following equations have been developed for drag and lift: Where CD and CL are the drag coefficient and the lift coefficient respectively. The net resistance force Fr is given in terms of an overall drag coefficient C Where: Fr = is resistance or drag force C= overall drag coefficient Ap = projected area normal to the direction of motion ρf = is mass density of fluid V = is relative velocity between main body of fluid and object. DRAG COEFFICIENT A number of relationships depending on the geometry of the body exist to compute the drag coefficient of agricultural materials. FOR LAMINAR FOR TURBULENT WHERE RE IS FLOW FLOW REYNOLDS NUMBER d is the effective dimension of the object (length, diameter etc..) η is the viscosity of the fluid.  TERMINAL VELOCITY The final constant speed of a free falling object at which the net gravitational accelerating force equals the resistance upward the drag force. Under the steady state condition where terminal velocity has been achieved: if the particle density is greater than the fluid density the particle will move downward If the particle density is smaller than the fluid density the particle will rise.  FORMULA FOR TERMINAL VELOCITY Where m = is mass of the particle g = is acceleration due to gravity ρp = is density of the particle ρf = is density of the fluid Ap = Projected area of the particle normal to the motion C = is drag coefficient  HYDRODYNAMIC PROPERTY The manner in which a fluid flows through a system is dependent upon the characteristics of the fluid, the size the shape and condition of the inside surface of the pipe or tube, and the fluid velocity. 2 TYPES: Laminar (streamline) Turbulent  TURBULENT FLOW LAMINAR FLOW the fluid moves in parallel the fluid moves in elemental elements swirls or eddies the direction of motion of each element being parallel to that of any other element. both velocity and direction of The velocity of any element is each element change with constant but not necessary the time same as of an adjacent element. violent mixing results no significant mixing results  CRITICAL VELOCITY Consider traverse of a fluid flowing in a pipe, it will show that the velocity is highest at the center and decreases towards the surface with the velocity at the surface being zero. This characteristic holds both for laminar and turbulent flow. For laminar flow, the velocity profile is parabolic in shape and the average velocity is one half the maximum which is at the center. For turbulent flow, the profile flattens and the relationship between the maximum and average velocity changes, its exact value being a function of a number of conditions under which flow results.  CRITICAL VELOCITY Defined as the velocity at which transitions occurs under which flows changes from laminar to turbulent Mathematical relationship represented by Reynolds number. REYNOLDS NUMBER where Re is Reynolds number d diameter of pipe (m) V average velocity of fluid (m/s) ρ fluid density (kg/m3) η dynamic viscosity of fluid (Pa s)  REYNOLDS NUMBER CONDITIONS For straight circular pipe with isothermal flow: If Re < 2130 the flow will be laminar If Re > 4000 the flow will be turbulent If Re is between 2130 to 4000, the characteristics of flow will depend upon the details of the structure and any defined prediction.  VISCOSITY the internal resistance of fluid to shear considered as the coefficient of friction of fluid in fluid Assuming two layers of fluid y meters apart, the inner space being filled with fluid, because of the resistance to motion offered by the fluid, a pressure P is required to maintain a constant velocity V of the top layer relative to the lower layer  FORMULA For most fluids the required force is directly proportional to the velocity gradients within the fluid, dV/dy Where η is the viscosity of the fluid (Ns/m2 ) or (Pas)  APPLICATION AIR CONVEYING SYSTEM Fluid bed dryers APPLICATION work on the principle of fluidization, a material is converted from a static solid-like state to a dynamic fluid-like state. hot gas or air is introduced through a perforated distribution plate into the area holding the material. This hot gas pumps through the spaces between solid particles. As the velocity of the gas or air increases, the upward forces on the particles increase, causing them to equal the gravitational forces below. This creates a state of fluidization where the particles are suspended in what appears to be a boiling bed of liquid. What once moved in a solid way can now flow like water. Each particle is in direct contact with, and surrounded by, the hot gas or air - creating an efficient and uniform drying process. SEPERATION OF FOREIGH MATERIALS  APPLICATION WASHING FOOD MATERIALS IN CONVEYOR BELT Thank You  PRACTICE PROBLEM Milk is flowing at 0.12m3/min in a 2.5 cm diameter pipe. If the temperature of the milk is 21 oC, is the flow turbulent or streamline? Viscosity of milk at 21 oC is 2.1x10-3 Pa-s and density is 1029kg/m3.  LABORATORY EXERCISE PROBLEM A cyclone 6 ft in diamter with an inlet 1 ft in diameter is designed as shown in figure with L = 2.5 d. The inlet is 4 in. wide, and particle inlet velocity is 50 ft/sec. The helix pitch is 15 degrees and the particle specific gravity is 1.2. a. Determine the smallest particle which can be collected b. Estimate the pressure drop through the unit.