Download presentation

Presentation is loading. Please wait.

Published byEfren Damon Modified over 2 years ago

2
Conduction & Convection

3
Quiz 9 – 2014.01.27 TIME IS UP!!! A flat furnace wall is constructed with a 4.5-inch layer of refractory brick (k = 0.080 Btu/ft·h· F) backed by a 9-inch layer of common brick (k = 0.800 Btu/ft·h· F) and a 2-inch layer of silica foam (k = 0.032 Btu/ft·h· F). The temperature of the inner face of the wall is 1200 F, and that of the outer face is 170 F. a.What is the temperature of the interface between the refractory brick and the common brick? b.What would be the temperature of the outer face if the silica foam is placed between the two brick layers?

4
Outline 2.Conduction Heat Transfer 2.1. Series/Parallel Resistances 2.2. Geometric Considerations 3.Convection Heat Transfer 3.1. Heat Transfer Coefficient 3.2. Dimensionless Groups for HTC Estimation

5
Geometric Considerations Heat Conduction Through Concentric Cylinders

6
Geometric Considerations Heat Conduction Through Concentric Cylinders

7
Geometric Considerations Heat Conduction Through Concentric Cylinders

8
Geometric Considerations Heat Conduction Through Hollow Spheres Integrating both sides:

9
Geometric Considerations Heat Conduction Through Hollow Spheres Rearranging:

10
Geometric Considerations Heat Conduction Through Hollow Spheres Define a geometric mean area: …and a geometric mean radius: *Final form

11
Shell Balance Plane Wall/Slab

12
Shell Balance Plane Wall/Slab

13
Shell Balance Plane Wall/Slab

14
Shell Balance Plane Wall/Slab

15
Shell Balance Cylinder

16
Shell Balance Sphere

17
Heat Transfer Coefficient Where: Q = heat flow rate A = heat transfer area h = heat transfer coefficient T w = temperature at solid wall T f = temperature at bulk fluid Convection Heat Transfer Useful Conversion:

18
Heat Transfer Coefficient Where: Q = heat flow rate A = heat transfer area h = heat transfer coefficient T w = temperature at solid wall T f = temperature at bulk fluid Convection Heat Transfer Driving force Thermal Resistance

19
Heat Transfer Coefficient

20
Dimensionless Groups Thermal conductivities are easy to determine by calorimetric experiments, but heat transfer coefficients require the analysis of transfer mechanisms. Mechanism Ratio Analysis 1. Heat, mass, and momentum transport are described by differential equations of change. e.g. Navier- Stokes Eq’n

21
Dimensionless Groups Thermal conductivities are easy to determine by calorimetric experiments, but heat transfer coefficients require the analysis of transfer mechanisms. Mechanism Ratio Analysis 2. However, these equations are complex and most of the time difficult to solve/integrate.

22
Dimensionless Groups Thermal conductivities are easy to determine by calorimetric experiments, but heat transfer coefficients require the analysis of transfer mechanisms. Mechanism Ratio Analysis 3. But still, valuable information is described in these equations, relating the different forces. Inertial Forces Pressure Forces Viscous Forces

23
Dimensionless Groups Thermal conductivities are easy to determine by calorimetric experiments, but heat transfer coefficients require the analysis of transfer mechanisms. Mechanism Ratio Analysis 4. In the Mechanism Ratio Analysis, solving the equations of change is replaced by empiricism. Inertial Forces Pressure Forces Viscous Forces

24
Dimensionless Groups Thermal conductivities are easy to determine by calorimetric experiments, but heat transfer coefficients require the analysis of transfer mechanisms. Mechanism Ratio Analysis 5. This is done by just taking the ratio of the mechanisms and making them into dimensionless groups. Inertial Forces Pressure Forces Viscous Forces Inertial Forces

25
Dimensionless Groups Inertial Forces Pressure Forces Viscous Forces Inertial Forces Reynolds Number, Re – the ratio of inertial to viscous forces. Euler Number, Eu – the ratio of pressure to inertial forces.

26
Dimensionless Groups Reynolds Number, Re – the ratio of inertial to viscous forces. Euler Number, Eu – the ratio of pressure to inertial forces. “If the phenomenon is so complex that negligible knowledge can be gained from the investigation of the differential equations, then empirical processes are available for evolving dimensionless groupings of the involved variables.” – Foust, 1980

27
Dimensionless Groups Useful dimensionless groups for Heat Transfer: Dim. GroupRatioEquation Prandtl, Pr molecular diffusivity of momentum / molecular diffusivity of heat Nusselt, Nu heat convection / heat conduction c P = specific heat (J/kg-K) μ = viscosity (Pa-s) D = characteristic length (diameter) (m) k = thermal conductivity (W/m-K) h = heat transfer coefficient (W/m 2 -K)

28
Dimensionless Groups Correlations for Heat Transfer Coefficients: Dittus-Boelter Equation Sieder-Tate Equation n = 0.4 when fluid is heated n = 0.3 when fluid is cooled (for forced convection/ turbulent, horizontal tubes) (for forced convection/ turbulent, Re > 10000 & 0.5 < Pr < 100)

29
Dimensionless Groups Exercise! An organic liquid enters a 0.834-in. ID horizontal steel tube, 3.5 ft long, at a rate of 5000 lb/hr. You are given that the specific heat, thermal conductivity, and viscosity of the liquid is 0.565 Btu/lb-°F, 0.0647 Btu/hr- ft-°F, and 0.59 lb/ft-hr, respectively. All these properties are assumed constant. If the liquid is being cooled, determine the inside-tube heat transfer coefficient using the Dittus-Boelter Equation.

Similar presentations

OK

Quiz – 2014.02.05 An organic liquid enters a 0.834-in. ID horizontal steel tube, 3.5 ft long, at a rate of 5000 lb/hr. You are given that the specific.

Quiz – 2014.02.05 An organic liquid enters a 0.834-in. ID horizontal steel tube, 3.5 ft long, at a rate of 5000 lb/hr. You are given that the specific.

© 2017 SlidePlayer.com Inc.

All rights reserved.

Ads by Google

Ppt on acid-base indicators experiment Ppt on acid rain Ppt on cross site scripting youtube Ppt on object-oriented programming concepts java Mp ppt online registration 2014 Project ppt on library management system Ppt on chapter 3 atoms and molecules 5th Ppt on properties of triangles for class 7th Ppt on science topics for class 9 Ppt on use of ms excel