{ "metadata": { "name": "", "signature": "sha256:5d41e84969ad0d553e20f8c382a4e66b5d40a08c263504221ff91368c27885dd" }, "nbformat": 3, "nbformat_minor": 0, "worksheets": [ { "cells": [ { "cell_type": "heading", "level": 1, "metadata": {}, "source": [ "Chapter 7 : Heat transfer by free convection" ] }, { "cell_type": "heading", "level": 2, "metadata": {}, "source": [ "Example 7.1 page : 129" ] }, { "cell_type": "code", "collapsed": false, "input": [ "# Variables\n", "tp = 200.; # Temperature of heated plate in degF\n", "ta = 60.; # Temperature of air in degF\n", "tf = (tp+ta)/2; # Temperature of film in degF\n", "delt = tp-ta; # Temperature difference in degF\n", "Z = 950000.; # As referred from the chart for corresponding temperature \n", "L = 18/12.; # Height of vertical plate in ft \n", "\n", "# Calculations \n", "X = L**3*(delt)*Z;\n", "# This value shows that it is laminar range so formula is as follows\n", "h = 0.29*(delt/L)**.25; # Heat transfer coeeficient in Btu/hr-ft**2-degF\n", "\n", "# Results\n", "print \"The film coefficient for free convetion for the heated plate is %.1f Btu/hr-ft**2/degF\"%(h)\n", "\n" ], "language": "python", "metadata": {}, "outputs": [ { "output_type": "stream", "stream": "stdout", "text": [ "The film coefficient for free convetion for the heated plate is 0.9 Btu/hr-ft**2/degF\n" ] } ], "prompt_number": 1 }, { "cell_type": "heading", "level": 2, "metadata": {}, "source": [ "Example 7.2 page : 131" ] }, { "cell_type": "code", "collapsed": false, "input": [ "\n", "# Variables\n", "tp = 300.; # Temperature of heated plate in degF\n", "ta = 80.; # Temperature of air in degF\n", "tf = (tp+ta)/2; # Temperature of film in degF\n", "delt = tp-ta; # Temperature difference in degF\n", "Z = 610000.; # As referred from the chart for corresponding temperature \n", "L = 7./12; # Height of vertical plate in ft \n", "\n", "# Calculations \n", "A = L*L; # Area of square plate in ft**2\n", "X = L**3*(delt)*Z;\n", "\n", "# This value shows that it is turbulent range , so formula for heat transfer coefficient is as follow \n", "h = 0.22*delt**(1./3); # Temperature coeeficient in Btu/hr-ft**2-degF\n", "q = h*A*delt; # Heat loss in Btu/hr\n", "\n", "# Results\n", "print \"The film coefficient for free convetion for the heated plate is %.2f Btu/hr-ft**2-degF\"%(h);\n", "print \" The heat loss by natural convection from the square plate is %.2f Btu/hr\"%(q);\n", "\n", "# book answer is wrong." ], "language": "python", "metadata": {}, "outputs": [ { "output_type": "stream", "stream": "stdout", "text": [ "The film coefficient for free convetion for the heated plate is 1.33 Btu/hr-ft**2-degF\n", " The heat loss by natural convection from the square plate is 99.42 Btu/hr\n" ] } ], "prompt_number": 3 }, { "cell_type": "heading", "level": 2, "metadata": {}, "source": [ "Example 7.3 page : 132" ] }, { "cell_type": "code", "collapsed": false, "input": [ "import math\n", "\n", "# Variables\n", "D = 0.375; # Outer diameter in ft\n", "T1 = 200.; # Pipe surface temperature in degF\n", "T2 = 70.; # Air temperature in degF\n", "Tf = (T1+T2)/2; # Film temperature at whih physical properties is to be measured\n", "delT = T1-T2;\n", "rho = 0.0667/32.2; # Density in slug/ft**3\n", "u = 0.0482/32.2; # Vismath.cosity in slug/ft-hr\n", "b = 1./(460+T2 );\n", "Cp = 0.241*32.2; # Heat capacity in Btu/slug-ft\n", "\n", "# Calculations \n", "# The value of specific heat is related to 1 lb mass so it must be multiplied to 32.2 to convert it into slugs\n", "k = 0.0164; # Thermal conductivity in Btu/hr-ft-degF\n", "g = 32.2*3600;\n", "# Unit of time used is hour so it must be converted to sec. Hence 3600 is multiplied \n", "Ngr = D**3*rho**2*b*g*delT/(u**3); # Grasshops number\n", "Npr = u*Cp/k; # Prandtls number\n", "A = math.log(Ngr*Npr);\n", "\n", "# Tha value of A is 6.866\n", "# Now seeing the value of nusselt number from the table\n", "Nnu = 25.2; # Nusselt number\n", "h = Nnu*k/D; # Heat transfer coefficient \n", "q = h*delT; # Heat loss per unit area in Btu/hr\n", "\n", "# Results\n", "print \"Heat loss per unit square foot is %d Btu/hr-ft**2\"%(q);\n" ], "language": "python", "metadata": {}, "outputs": [ { "output_type": "stream", "stream": "stdout", "text": [ "Heat loss per unit square foot is 143 Btu/hr-ft**2\n" ] } ], "prompt_number": 4 }, { "cell_type": "heading", "level": 2, "metadata": {}, "source": [ "Example 7.4 page : 134" ] }, { "cell_type": "code", "collapsed": false, "input": [ "\n", "# Variables\n", "tp = 200.; # Temperature of heated plate in degF\n", "ta = 70.; # Temperature of air in degF\n", "tf = (tp+ta)/2; # Temperature of film in degF\n", "delt = tp-ta; # Temperature difference in degF\n", "Z = 910000.; # As referred from the chart for corresponding temperature \n", "D = 4.5/12; # Diameter of pipe in ft\n", "\n", "# Calculations \n", "X = D**3*(delt)*Z;\n", "# This value lies between X = 1000 to X = 10**9 , so formula for heat transfer coefficient is as follow \n", "\n", "h = 0.27*(delt/D)**(1./4); # Temperature coeeficient in Btu/hr-ft**2-degF\n", "q = h*delt; # Heat loss in Btu/hr\n", "\n", "# Results\n", "print \"The film coefficient for free convetion for the heated plate is %.2f Btu/hr-ft**2-degF\"%(h);\n", "print \" The heat loss by natural convection from the square plateis %d Btu/hr\"%(q);\n" ], "language": "python", "metadata": {}, "outputs": [ { "output_type": "stream", "stream": "stdout", "text": [ "The film coefficient for free convetion for the heated plate is 1.17 Btu/hr-ft**2-degF\n", " The heat loss by natural convection from the square plateis 151 Btu/hr\n" ] } ], "prompt_number": 5 } ], "metadata": {} } ] }