HEAT TRANSFER SYMBOLS AND FORMULAS1
https://www.mesubjects.net/wpadmin/post.php?post=7462&action=edit Dimensionless num HT
https://www.mesubjects.net/wpadmin/post.php?post=7420&action=edit HT Symbols2
https://www.mesubjects.net/wpadmin/post.php?post=7368&action=edit MCQ HT1
https://www.mesubjects.net/wpadmin/post.php?post=7340&action=edit MCQ HT2
https://www.mesubjects.net/wpadmin/post.php?post=6732&action=edit PTU HT Paper Sol B
https://www.mesubjects.net/wpadmin/post.php?post=6434&action=edit PTU HT Paper Sol A
https://www.mesubjects.net/wpadmin/post.php?post=6261&action=edit Fin HT
_{HEAT TRANSFER SYMBOLS AND FORMULAS1}
_{Heat transfer symbols and formula are very helpful in understanding the topic in great depth with lot of ease.}
_{Sr. No.} 
_{Item or quantity} 
_{Definition} 
_{Symbol} 
_{Formula} 
_{Units} 
_{1.} 
_{Thermal conductivity} 
_{Rate of heat transfer per unit area per unit temperature difference and per unit wall thickness.}_{k=q. For A= 1m2 , dt=10c and dx=1m} 
_{k} 
_{Q=kA∂t/∂x} 
_{W/m0C} 
_{2.} 
_{Thermal diffusivity} 
_{Ratio of thermal conductivity to heat capacity per unit volume} 
_{α} 
_{Α =k/ρcp} 
_{m2/s} 
_{3.} 
_{Temperature gradient} 
_{Change of temperature with respect to x,}_{it is NEGATIVE because as x increases t decreases.} 
_{∂t/∂x,.} 
_{∂t/∂x,.} 
_{0C/m} 
_{4.} 
_{Fourier equation} 
_{Gives rate of heat transfer in conduction.} 
_{q. = k A dt/dx} 
_{q. = k A dt /dx} 
_{WATTS} 
_{5.} 
_{Fourier Law} 
_{(i) q. A}_{(ii) q. dt}_{(iii) q. A dt}_{(iv) q. =h A dt} 
_{q. =h A dt} 
_{q. =h A dt} 
_{W} 
_{6.} 
_{CONDUCTION}_{CONVECTION}_{RADIATION} 
_{Fourier equation is conduction equation.}_{Newton’s Law of cooling is convection equation.}_{Stephen’s Boltzmann Law is radiation equation.} 

_{7.} 
_{CRITICAL RADIUS OF INSULATION} 
_{It is a radius of insulation at which the rate of heat transfer is maximum} 
_{rcr} 
_{For a cylinder}_{rcr = k/h0}_{For a sphere}_{rcr =2 k/h0} 
_{mm} 
_{8.} 
_{Biot number} 
_{internal resistance/external resistance}_{=Conductive resistance/Convective resistance} 
_{Bi} 
_{Bi =hx/ksolid} 
_{No units} 
_{9.} 
_{Steady state} 
_{Temperature does change with time. Human body} 
_{∂t/∂time=0} 
_{∂t/∂time=0} 
_{0C/s} 
_{10.} 
_{Unsteady state} 
_{Temperature changes with time. Atmospheric temp} 
_{∂t/∂time} 
_{∂t/∂time} 
_{0C/s} 
_{11.} 
_{FREE OR NATURAL CONVECTION} 
When bulk motion of the fluid is caused by the density difference or Buoyancy force, it is called Free or Natural convection. It is governed by the product of Grashoff’s number and Prandtl number. All the convection processes in nature are of free convection.

q^{.} = h A dt  q^{.} = h A dt 
12.  FORCED CONVECTION  Motion of liquid is caused by a pump and the motion of gas is caused by a blower over the heated surface——–Car radiator. It is governed by the Reynolds number and Prandtl number.  q^{.} = h A dt  q^{.} = h A dt 
13 
CRITICAL REYNOLD NUMBER 
Reynolds number where the laminar region ends. (a)Its value is 5 x 10^{5}for a flat horizontal plate. (b) It is 2100 for flow through a pipe.

Re  Re = ρVD/μ 
14  Nusselt number  It is used to find ‘h’  Nu  Nu= hl/k_{fluid} 
15  OVERALL HEAT TRANSFER COEFFICIENT  It accounts for convection+conduction+convection  U  
16  LMTD  It is a mean temperature DIFFERENCE for a heat exchanger  LMTD  LMTD = (θ_{max}—θ_{min})/ ln(θmax/θ_{min}) 
17  NTU  It is number of transfer units. It represents area.  NTU  NTU= U A/ C_{min}

18.  Effectiveness of a HEX  Ratio of actual RATE of heat transfer to RATE of maximum heat transfer.
q^{. }_{actual} = m_{h} c_{ph} dt_{h} = m_{c} c_{pc} dt_{c} q^{.}_{max} = C_{min}(t_{hot in} – t_{cold in})

Є  Є = q^{.}_{actual }/q^{.}_{max} 
19.  FIN  FIN IS AN EXTENDED SURFACE WHICH INCREASES SIGNIFICANTLY THE SURFACE AREA AS WELL AS THE RATE OF HEAT TRANSFER MOST ECONOMICALLY.  —  — 
20.  FIN EFFICIENCY  η_{f }= actual q^{.}_{fin}/ q^{.}_{max}
q^{.}_{max } is rate of HT with same base temperature t_{b} all along the fin length

η_{f }  η_{f }= (Pk/hA_{c})^{1/2} 
21.  FIN EFFECTIVENESS  Ratio of rate of heat transfer with fin to rate of heat transfer without fin.  Є_{f}  Є_{f} = q^{.}_{with fin}/ q^{.}_{without fin}
_{ } Є_{f}_{=}(kP/hA_{c})^{1/2} _{ } 
22.  Dimensional analysis  It is a process to develop an equation between dimensionless numbers based on dimensional homogenity  –  – 
23  Bukingham
Theorem 
Let total number of variables =n
Total no. of dimensions = m Number of 
–  – 
24.  Planck’s Law  Gives emissive power of a black body FOR A SINGLE WAVELENGTH  E_{λ} = C_{1}λ^{—5}/(e^{c2/λT} –1)  E_{λ} = C_{1}λ^{—5}/(e^{c2/λT} –1) 