Radiation Heat Transfer

-
Image
View Factor Orientation (or View factor or shape factor) plays an important role in radiation heat transfer. View factor is defined as, "fraction of radiation leaving surface 'i' and strike 'j' ". Summation Rule (View Factor) If there is are similar surfaces 'i' and 'j' , then: Blackbody Radiation Exchange Radiation Exchange between Opaque, Diffuse, Gray surfaces in an Enclosure 1. Opaque 2. Surfaces 3. Two surface enclosure Radiation Shield It is used to protect surfaces from radiation act like a reflective surface. References: Material from Class Lectures + Book named Fundamentals of Heat and Mass Transfer by Theodore L. Bergman + My knowledge.  Photoshoped pics  are developed.  Some pics and GIF from Google.   Videos from YouTube ( Engineering Sights ).
Post a Comment

Flow Over Immersed Bodies_B

-

Momentum Integral Boundary Layer Equation for a Flat Plate

  • Karman Solution (alternate method)  ↠  approx. closed form solution.
  • Boundary layer theory  ↠  determination of drag (due to shear force)  ↠  using differential equation for laminar boundary layer flow (no exact solution)  ↠  alternate approx. method.
Ques: How to solve Boundary layer?
There are two solutions:
  1. By Blasius Solution   ↠   limited to laminar boundary layer and for a flat plate only (without a pressure gradient).
  2. By Momentum Integral Equation   ↠   used to obtain approx. information on boundary layer growth for the general case (laminar or turbulent boundary layers with or without pressure gradient).
The flow entering the CV at the leading edge of the plate is uniform while the velocity of the flow leaving the CV varies from the upstream velocities at the edge of the boundary layer to zero velocity on plate.
  • Drag on a flat plate is related to the momentum deficit within the boundary layer.
  • For a Laminar layer flow  ↠  CDF = CF which is a function of Reynold's number (Surface roughness is not important).
  • For Turbulent flow, surface roughness affects the shear stress (friction drag coefficient).

Transition from Laminar To Turbulent Flow

Complex process of transition from laminar to turbulent flow is due to the instability of the flow field. Parameter that governs the transition to turbulent flow is Reynold's number based on the distance from leading edge of plate.
  • Reynold's number at Transition location = Function (Surface roughness, Surface curvature, Disturbance in the flow outside the boundary layer).
  • If flat plate with sharp leading edge transition take place at x from the leading edge  ↠  Rexcr = 2e5 (mostly used) to 3e6.
  • Transition occur over a region (not at single point)  ↠  Rex = Rexcr.

Turbulent Boundary Layer on a Flat Plate

  • Drag coefficient diagram (boundary layer flow)   ↠   Moody diagram.
  • Pressure & Viscous forces balanced (pipe flow), fluid inertia remains constant  ↠  for Fully developed Horizontal pipe flow.
  • Boundary Layer flow (Horizontal pipe)  ↠  Inertia & Viscous effects balanced, pressure pipe flow.

Boundary Layer with Pressure Gradient


In a situation where pressure increases downstream the fluid particles can move up against it by virtue of their K.E.
  • Flow Reversal  ↠  inside the B.L. the velocity in a layer could reduce so much that k.e. of the fluid particles is no longer adequate to move the particles against the pressure gradient.
  • Flow separation  ↠  the fluid layer higher up still have energy to move forward a rolling of fluid stream occurs.
  • For laminar  ↠  separation at 90-degrees and drag is much high.
  • For turbulent  ↠  separation after 90-degrees (at 100-135 degrees).
  • Because of B.L. separation, the average pressure on the rear half of cylinder is less than that on the front half.
Note:

References:

  • Material from Class Lectures + Book named Fundamentals of Fluid Mechanics by Munson, Young & Okiishi's (8th Edition) + my knowledge. 
  • Pics and GIF from Google Images.  
  • Videos from YouTube (Engineering Sights).

Comments

Post a Comment

HI, we wI'll contact you later

Popular posts from this blog

Corrections Of Tape

-
Image
TAPING CORRECTIONS There are two types of corrections depending upon the type of errors in tape due to the different conditions.  1. Systematic Errors : Slope Erroneous tape length Temperature Tension Sag 2. Random Errors : Slope  Alignment Marking & Plumbing Temperature Tension & Sag 1. Temperature Correction  It is necessary to apply this correction, since the length of a tape is increased as its temperature is raised, and consequently, the measured distance is too small.  It is given by the formula,                         C t  = 𝛼 (T m  – T o )L Where,   C t  = the correction for temperature, in m.      𝛼 = the coefficient of thermal expansion.    T m  = the mean temperature during measurement.          T o  = the tempe...
Read more

Orthographic Projections

-
Image
Projection: The term Projection is defined as: Presentation of an image or an object on a surface. The principles used to graphically represent 3-D objects and structures on 2-D media and it based on two variables: Line of Sight.  Plane of Projection.  Line of Sight & Plane of Projection: Line of sight is divided into 2 types: Parallel Projection Converging Projection  & A plane of projection is an imaginary flat plane upon which the image created by the lines of sight is projected. Orthographic Projection: When the projectors are parallel to each other and perpendicular to the plane of projection. The lines pf sight of the observer create a view on the screen. The screen is referred to as the Plane of Projection (POP). The lines of sight are called Projection lines or projectors. Rules of Orthographic Projection: Edges that are parallel to a plane of projection appear as lines. Edges that are incl...
Read more

Diffusion

-
Image
Diffusion  It is defined as, " The transportation of one atom from one state into other state " . Used for hardening of surfaces like die parts, gear, bolt, machine element. Hardening can be done using Temperature-Heat treatment ( diffusion couple ). There are two types of diffusion: Inter-diffusion   ↔ in which atoms tend to migrate from region of high concentration to low concentration . Self-diffusion   ↔  in which atoms migrate within a solid. Diffusion Mechanism There can be two conditions of moving atoms Empty spaces between adjacent sites. Atoms have sufficient energy to break bond with neighbors atoms which cause lattice distortion during displacement. If we rise temperature, small fractions of atoms is capable of diffusive motion (which cause lattice distortion). Types of Diffusion 1. Vacancy Diffusion It is defined as, " interchange of an atom from a normal lattice position to the adjacent vacant lattice site " . Extent of vacancy diffusion is a function...
Read more