Free Convection and the Rayleigh Number


Here we discuss the role of Rayleigh number on the initiation of convection.

Continuing with our convection discussion from the introduction of the phenomenon and the subsequent explanation for the mechanism of free convection , we shall discuss here under what conditions free convection is initiated in an enclosed fluid.

At the end of the post on the mechanism of free convection we realized that free convection should be observed in a fluid region whenever there is a temperature gradient, however small it may be. But such sensitive dependence of the initiation of the flow on the temperature gradient is not observed in actual circumstances. The onset of buoyancy driven free convection in an enclosed fluid has to take into consideration two more modes of energy dissipation in the fluid.

The pressure and buoyancy force imbalance equation, which explains the convection motion, needs to be recast to accommodate two more forces. One of our initial assumptions while explaining the mechanism of buoyancy driven convection is that before the temperature gradient prevails the fluid is at rest and is not subjected to any external influence which might induce motion. So when the fluid tries to move, or circulate (i.e., convect), it does so with minimum velocity. When the fluid packet moves, its motion is impeded by the viscous drag between the packet and the surrounding larger fluid bulk.

Viscosity, as we know, is the internal fluid resistance offered to a change in the momentum. For any fluid it can be evaluated from the constitutive relation

\mu = \tau \cdot \frac{du}{dy}~\cdots (1)

The above ‘equation’ means, dynamic viscosity mu is equal to the ratio of the applied shear stress Tau on the fluid and the perpendicular direction change in the velocity component of the fluid (du/dy ) . If the fluid could be imagined as a stack of newspaper loosely tied together, when the top paper is ‘pushed’ along its surface, the stack would react to the ’shear’, which is akin to the resistance offered by the fluid to the shear stress and is attributed to the fluid’s property viscosity.In our fluid packet, this viscous resistance acts against the buoyancy force and tries to impede motion. If the magnitude of the viscous drag force equals that of the buoyancy force, motion will cease.

The second dissipative effect is from the fact that Convection is not the only mode of heat transfer that could happen in the given circumstance – Conduction and Radiation being the other two. Out of these two, Radiative effects are predominant only at very large temperature values but Conductive heat transfer cannot be altogether ignored. Since the hot fluid packet from the bottom is displaced up by the buoyancy force into a colder region of fluid, the hot packet can ‘leak’ energy as heat into the colder surrounding, because of the temperature difference.

To explain in another way, the microscopic definition of heat assumes the molecules in the warm packet to have higher average velocity than that of the surrounding. This makes the molecules in the packet to jiggle more freely thereby exchanging energy with the surrounding (colder) molecules of lesser velocity resulting in the equalisation of their velocities. This results ultimately in the premature cooling of the fluid packet that began to raise from the bottom. For the fluid packet coming down from a cooler environment the heat transfer is in the other way leading to similar results.

So if the local temperature difference (say, between the fluid packet and its surrounding fluid) is reduced by heat diffusion (conduction) it results in a reduction in the buoyancy force. It is necessary that the buoyancy force, which is the result of the temperature gradient (because of the dependence of density on temperature, as we saw), must exceed the dissipative forces of viscous drag and heat diffusion to ensure the onset of\ convective flow.

Hence, for the fluid to convect, the buoyancy force, resulting in the displacement of the fluid packet up and down, must be more than the magnitude of the ‘fluid brake’ and ‘heat diffusion’. These requirement are expressed as a non-dimensional number, called the Rayleigh Number in honour of Lord Rayleigh who came up with the explanation for this convection behaviour of an enclosed fluid subjected to a temperature differential.

The Rayleigh number is the buoyant force divided by the product of the viscous drag and the rate of heat diffusion. In equation form using symbols it reads

Ra = \frac{g\beta \Delta T H^3}{\alpha \nu}~\cdots (2)

where Beta is the coefficient of thermal expansion of the fluid, Delta T is the temperature difference between the bottom hot and top cold end in the figure separated by height H, Alpha is the thermal diffusivity of the fluid and Nu the kinematic viscosity (dynamic viscosity Mu divided by the density) of the fluid.

Convection sets in when the Rayleigh number exceeds a certain critical value. Thus Rayleigh Number (a non-dimensional number, if you notice) is a quantitative measure or representation of when the switch from conductive to convective transport happens for a given fluid plus geometry configuration. Once the Ra exceeds a critical value, henceforth, the dominant energy transport mechanism in the fluid would be convection.

Lord Rayleigh’s analysis of the problem of convective flow was initiated by the 1901 experiments of Henri Benard. It can be found with all of the minute derivation details, in Subrahmanyam Chandrashekar’s book Hydrodynamic and Hydromagnetic Stability, Dover Publishers.

While trying to explain those experiments of Benard, Rayleigh devised the theory explained above. For instance, for a thin fluid layer (H is very small when compared to the length) confined between two sufficiently long horizontal parallel plates, with the bottom plate hotter than the top one by a ?T, it would take the Ra to be greater than 1708 for convection motion to set in the fluid. For any fluid (water or air or mercury or, you get the idea…). This requirement of Ra > 1708 for the above configuration has been shown experimentally to be true over the years by many researchers. For interested readers, more on these experiments can be read from the book by A. V. Getling, Rayleigh Benard Convection: Structures and Dynamics , 1998, World Scientific publishers.

I have also done this experiment.

And so have you, in a slightly modified form, when you made hot water using a stove (not in microwave oven – that is a different type of convection).