Grashof number

Heat Transfer

Free convection is caused by a change in density of a fluid due to a temperature change or gradient. Usually the density decreases due to an increase in temperature and causes the fluid to rise. This motion is caused by the buoyancy force. The major force that resists the motion is the viscous force. The Grashof number is a way to quantify the opposing forces.

The Grashof number is:

G r L = g β ( T s − T ∞ ) L 3 ν 2 for vertical flat plates G r D = g β ( T s − T ∞ ) D 3 ν 2 for pipes G r D = g β ( T s − T ∞ ) D 3 ν 2 for bluff bodies

where:

g is acceleration due to Earth's gravity β is the coefficient of thermal expansion (equal to approximately 1/T, for ideal gases) T_s is the surface temperature T_∞ is the bulk temperature L is the vertical length D is the diameter ν is the kinematic viscosity.

The L and D subscripts indicate the length scale basis for the Grashof Number.

The transition to turbulent flow occurs in the range 10⁸ < Gr_L < 10⁹ for natural convection from vertical flat plates. At higher Grashof numbers, the boundary layer is turbulent; at lower Grashof numbers, the boundary layer is laminar.

Mass transfer

There is an analogous form of the Grashof number used in cases of natural convection mass transfer problems. In the case of mass transfer, natural convection is caused by concentration gradients rather than temperature gradients.

G r c = g β ∗ ( C a , s − C a , a ) L 3 ν 2

where:

β ∗ = − 1 ρ ( ∂ ρ ∂ C a ) T , p

and:

g is acceleration due to Earth's gravity C_a,s is the concentration of species a at surface C_a,a is the concentration of species a in ambient medium L is the characteristic length ν is the kinematic viscosity ρ is the fluid density C_a is the concentration of species a T is the temperature (constant) p is the pressure (constant).

Relationship to Other Dimensionless Numbers

The Rayleigh number, shown below, is a dimensionless number that characterizes convection problems in heat transfer. A critical value exists for the Rayleigh number, above which fluid motion occurs.

R a x = G r x P r

The ratio of the Grashof number to the square of the Reynold's number may be used to determine if forced or free convection may be neglected for a system, or if there's a combination of the two. If the ratio is much less than one, then free convection may be ignored. If the ratio is much greater than one, forced convection may be ignored. Otherwise, the regime is combined forced and free convection.

G r R e 2 >> 1 forced convection may be ignored

G r R e 2 ≈ 1 combined forced and free convection

G r R e 2 << 1 free convection may be neglected

Derivation

The first step to deriving the Grashof number is manipulating the volume expansion coefficient, β as follows.

β = 1 v ( ∂ v ∂ T ) p = − 1 ρ ( ∂ ρ ∂ T ) p

It should be noted that the v in the equation above, which represents specific volume, is not the same as the v in the subsequent sections of this derivation, which will represent a velocity. This partial relation of the volume expansion coefficient, β , with respect to fluid density, ρ , given constant pressure, can be rewritten as

ρ = ρ o ( 1 − β Δ T )

where:

ρ o is the bulk fluid density ρ is the boundary layer density Δ T = ( T − T o ) , the temperature difference between boundary layer and bulk fluid.

There are two different ways to find the Grashof number from this point. One involves the energy equation while the other incorporates the buoyant force due to the difference in density between the boundary layer and bulk fluid.

Energy Equation

This discussion involving the energy equation is with respect to rotationally symmetric flow. This analysis will take into consideration the effect of gravitational acceleration on flow and heat transfer. The mathematical equations to follow apply both to rotational symmetric flow as well as two-dimensional planar flow.

∂ ∂ s ( ρ u r o n ) + ∂ ∂ y ( ρ v r o n ) = 0

where:

s is the rotational direction, i.e. direction parallel to the surface u is the tangential velocity, i.e. velocity parallel to the surface y is the planar direction, i.e. direction normal to the surface v is the normal velocity, i.e. velocity normal to the surface r o is the radius.

In this equation the superscript n is to differentiate between rotationally symmetric flow from planar flow. The following characteristics of this equation hold true.

n = 1: rotationally symmetric flow n = 0: planar, two-dimensional flow g is gravitational acceleration

This equation expands to the following with the addition of physical fluid properties:

ρ ( u ∂ u ∂ s + v ∂ u ∂ y ) = ∂ ∂ y ( μ ∂ u ∂ y ) − d p d s + ρ g .

From here we can further simplify the momentum equation by setting the bulk fluid velocity to 0 (u = 0).

d p d s = ρ o g

This relation shows that the pressure gradient is simply a product of the bulk fluid density and the gravitational acceleration. The next step is to plug in the pressure gradient into the momentum equation.

ρ ( u ∂ u ∂ s + v ∂ u ∂ y ) = μ ( ∂ 2 u ∂ y 2 ) + ρ g β ( T − T o )

Further simplification of the momentum equation comes by substituting the volume expansion coefficient, density relationship ρ o − ρ = β ρ ( T − T o ) , found above, and kinematic viscosity relationship, ν = μ ρ , into the momentum equation.

u ( ∂ u ∂ s ) + v ( ∂ v ∂ y ) = ν ( ∂ 2 u ∂ y 2 ) + g β ( T − T o )

To find the Grashof Number from this point, the preceding equation must be non-dimensionalized. This means that every variable in the equation should have no dimension and should instead be a ratio characteristic to the geometry and setup of the problem. This is done by dividing each variable by corresponding constant quantities. Lengths are divided by a characteristic length, L c . Velocities are divided by appropriate reference velocities, V , which, considering the Reynolds number, gives V = R e L ν L c . Temperatures are divided by the appropriate temperature difference, ( T s − T o ) . These dimensionless parameters look like the following:

s ∗ = s L c , y ∗ = y L c , u ∗ = u V , v ∗ = v V , T ∗ = ( T − T o ) ( T s − T o ) .

The asterisks represent dimensionless parameter. Combining these dimensionless equations with the momentum equations gives the following simplified equation.

u ∗ ∂ u ∗ ∂ s ∗ + v ∗ ∂ u ∗ ∂ y ∗ = [ g β ( T s − T o ) L c 3 ν 2 ] T ∗ R e L 2 + 1 R e L ∂ 2 u ∗ ∂ y ∗ 2

where:

T s is the surface temperature T o is the bulk fluid temperature L c is the characteristic length.

The dimensionless parameter enclosed in the brackets in the preceding equation is known as the Grashof Number:

G r = g β ( T s − T o ) L c 3 ν 2 .

Buckingham Pi Theorem

Another form of dimensional analysis that will result in the Grashof number is known as the Buckingham Pi theorem. This method takes into account the buoyancy force per unit volume, F b due to the density difference in the boundary layer and the bulk fluid.

F b = ( ρ − ρ o ) g

This equation can be manipulated to give,

F b = β g ρ o Δ T .

The list of variables that are used in the Buckingham Pi method is listed below, along with their symbols and dimensions.

With reference to the Buckingham Pi Theorem there are 9 – 5 = 4 dimensionless groups. Choose L, μ , k, g and β as the reference variables. Thus the π groups are as follows:

π 1 = L a μ b k c β d g e c p , π 2 = L f μ g k h β i g j ρ , π 3 = L k μ l k m β n g o Δ T , π 4 = L q μ r k s β t g u h .

Solving these π groups gives:

π 1 = μ ( c p ) k = P r , π 2 = l 3 g ρ 2 μ 2 , π 3 = β Δ T , π 4 = h L k = N u

From the two groups π 2 and π 3 , the product forms the Grashof number:

π 2 π 3 = β g ρ 2 Δ T L 3 μ 2 = G r .

Taking ν = μ ρ and Δ T = ( T s − T o ) the preceding equation can be rendered as the same result from deriving the Grashof number from the energy equation.

G r = β g Δ T L 3 ν 2

In forced convection the Reynolds number governs the fluid flow. But, in natural convection the Grashof number is the dimensionless parameter that governs the fluid flow. Using the energy equation and the buoyant force combined with dimensional analysis provides two different ways to derive the Grashof number.