Thermal diffusivity

In <a href="/facts/Thermodynamics/q4hIx3yP">thermodynamics</a>, thermal diffusivity is the <a href="/facts/Thermal_conductivity/qWnSG3nQ">thermal conductivity</a> divided by <a href="/facts/Density/92p6hh9I">density</a> and <a href="/facts/Specific_heat_capacity/9P0U6YgI">specific heat capacity</a> at constant pressure. It is a measure of the rate of <a href="/facts/Heat_transfer/1IV3WbFL">heat transfer</a> inside a material and has <a href="/facts/SI/NShmfzlD">SI units</a> of m2/s. It is an <a href="/facts/Intensive_property/UcgSh8EG">intensive property</a>. Thermal diffusivity is usually denoted by lowercase <a href="/facts/Alpha/I51VqVVm">alpha</a> (α), but a, h, κ (<a href="/facts/Kappa/UHoskwDQ">kappa</a>), K, D, 
 
 
 
 
 D
 
 T
 
 
 
 
 {\displaystyle D_{T}}
 
 are also used.
The formula is

α
        =
        
          
            k
            
              ρ
              
                c
                
                  p
                
              
            
          
        
        ,
      
    
    {\displaystyle \alpha ={\frac {k}{\rho c_{p}}},}

where

k is <a href="/facts/Thermal_conductivity/qWnSG3nQ">thermal conductivity</a> (W/(m·K)),
cp is <a href="/facts/Specific_heat_capacity/9P0U6YgI">specific heat capacity</a> (J/(kg·K)),
ρ is <a href="/facts/Density/92p6hh9I">density</a> (kg/m3).
Together, ρcp can be considered the <a href="/facts/Volumetric_heat_capacity/utfQ52xj">volumetric heat capacity</a> (J/(m3·K)).
Thermal diffusivity is a positive <a href="/facts/Coefficient/5Hwq2Wuq">coefficient</a> in the <a href="/facts/Heat_equation/2JPPr5GG">heat equation</a>:

∂
              T
            
            
              ∂
              t
            
          
        
        =
        α
        
          ∇
          
            2
          
        
        T
        .
      
    
    {\displaystyle {\frac {\partial T}{\partial t}}=\alpha \nabla ^{2}T.}

One way to view thermal diffusivity is as the ratio of the <a href="/facts/Time_derivative/AWV28T13">time derivative</a> of <a href="/facts/Temperature/QyGe4gmj">temperature</a> to its <a href="/facts/Second_derivative/3aA6j4EY">curvature</a>, quantifying the rate at which temperature concavity is "smoothed out". In a substance with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its energy storage capacity or "thermal bulk".
Thermal diffusivity and <a href="/facts/Thermal_effusivity/J3PUmxtV">thermal effusivity</a> are related concepts and quantities used to simulate <a href="/facts/Non-equilibrium_thermodynamics/SvcDqcqm">non-equilibrium thermodynamics</a>. Diffusivity is the more fundamental concept and describes the <a href="/facts/Stochastic_process/Ng1n1dnB">stochastic process</a> of heat spread throughout some <a href="/facts/Local_property/sdBEwIy2">local</a> volume of a substance. Effusivity describes the corresponding transient process of heat flow through some local area of interest. Upon reaching a <a href="/facts/Steady_state/qgrL4BNs">steady state</a>, where the stored energy distribution stabilizes, the thermal conductivity (k) may be sufficient to describe heat transfers inside solid or rigid bodies by applying <a href="/facts/Fourier%27s_law/5vRktgcS">Fourier's law</a>.
Thermal diffusivity is often measured with the <a href="/facts/Laser_flash_analysis/teH4eloG">flash method</a>. It involves heating a strip or cylindrical sample with a short energy pulse at one end and analyzing the temperature change (reduction in amplitude and phase shift of the pulse) a short distance away.

Thermal diffusivity open-in-new

Thermal diffusivity