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{
"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": {}
}
]
}