Prandtl number signifies the thickness of thermal boundary layer and thickness of hydrodyanamic boundary layer, depending on whether it is equal to one, or more than one or less than one. Tells us the relative thickness of thermal boundary layer to momentum boundary layer.
In fluid dynamics, the Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid. Convection includes both advection (fluid motion) and diffusion (conduction). A Nusselt number of value one represents heat transfer by pure conduction.
268): “Then the Nusselt number may be interpreted as the ratio of heat transfer by convection to conduction across the fluid layer of thickness L. A larger value of the Nusselt number implies enhanced heat transfer by convection.”
- Nusselt Number : Nu = hL/k.
- Convection Heat Transfer Coefficient : k = Nuk/L.
- Characteristic Length : L = Nuk/h.
- Thermal Conductivity of the Fluid : k = hL/Nu.
- Where, Nu = Nusselt Number, h = Convection Heat Transfer Coefficient, L = Characteristic Length, k = Thermal Conductivity of the Fluid.
All pure numbers are dimensionless quantities, for example 1, i, π, e, and φ. Units of number such as the dozen, gross, googol, and Avogadro's number may also be considered dimensionless.
The Prandtl number (Pr = ν / α) is defined as the dimensionless ratio between kinematic viscosity (ν) and thermal diffusivity, α = k / (ρ·cp); where k stands for thermal conductivity, ρ stands for density, and cp is the (isobaric) specific heat capacity.
The Grashof number (Gr) is a dimensionless number in fluid dynamics and heat transfer which approximates the ratio of the buoyancy to viscous force acting on a fluid. It frequently arises in the study of situations involving natural convection and is analogous to the Reynolds number.
Dimensionless Numbers
Reynolds number: R e = ℒ U / ν represents the ratio between inertial and viscous forces. It is mainly used to define the transition from laminar to turbulent flow. At the process scale, the Reynolds number can be very large (> 106).The significance of the Grashof number is that it represents the ratio between the buoyancy force due to spatial variation in fluid density (caused by temperature differences) to the restraining force due to the viscosisty of the fluid.
Nusselt number is required to find 'h' which is convective heat transfer coefficient. The physical interpretation of Nusselt number is the enhancement of heat transfer due to convection over conduction alone. If Nu=1, then, than your fluid is stationary and all heat transfer is by conduction.
Thus, a larger Reynold's number will definitely impact the rate of heat transfer in an external forced convection process. This is not exactly the same for internal forced convection. With internal forced convection, the Reynold's number will effect the hydrodynamic and thermal entry regions.
Reynold number is a very important dimensionless quantity in fluid mechanics. It's defined as the ratio of inertial forces to viscous forces to predict the fluid flow conditions. For example, it can be used to characterize different flow regimes within a similar fluid, such as laminar or turbulent flow.
The Thermal Boundary Layer is a region of a fluid flow, near a solid surface, where the fluid temperatures are directly influenced by heating or cooling from the surface wall. The two boundary layers may be expected to have similar characteristics but do not normally coincide.
Examples of Convection
- Boiling water - The heat passes from the burner into the pot, heating the water at the bottom.
- Radiator - Puts warm air out at the top and draws in cooler air at the bottom.
- Steaming cup of hot tea - The steam is showing heat being transfered into the air.
- Ice melting - Heat moves to the ice from the air.
Examples for forced convection are as follows: Using a fan on a hot summer day. Explanation: the sweat that our body produces is for effective heat transfer. So when the fan is off, the air around us absorbs the water vapour until its saturated.
There are three types of heat transfer: conduction, convection and radiation. Convection is a type of heat transfer that can only happen in liquids and gases, because it involves those liquids or gases physically moving. Convection happens when there is a difference in temperature between two parts of a liquid or gas.
Everyday Examples of Convection
Boiling water - The heat passes from the burner into the pot, heating the water at the bottom. Then, this hot water rises and cooler water moves down to replace it, causing a circular motion. Hot air balloon - A heater inside the balloon heats the air and so the air moves upward.Natural convection arises from temperature differences among air parcels, or heat transfer at surfaces (i.e. surface-to-air temperature difference). In the absence of forced convection, natural convection becomes the only means of air mixing inside enclosed spaces.
Thermal Convection
There are two types of convection: natural convection and forced convection. Natural convection is produced by density differences in a fluid due to temperature differences (e.g., as in “hot air rises”).Everyday Examples of Convection
Boiling water - The heat passes from the burner into the pot, heating the water at the bottom. Then, this hot water rises and cooler water moves down to replace it, causing a circular motion. Radiator - Puts warm air out at the top and draws in cooler air at the bottom.The convection caused by winds is natural convection for the earth, but it is forced convection for bodies subjected to the winds since for the body it makes no difference whether the air motion is caused by a fan or by the winds.
Convection is called forced convection if the fluid is forced to flow over the surface by external means such as a fan, pump, or the wind. In Free (or natural) convection, the flow is induced by buoyancy forces, which are due to density differences caused by temperature variations in the fluid.
Convection occurs when heat is carried away from your body via moving air. If the surrounding air is cooler than your skin, the air will absorb your heat and rise. As the warmed air rises around you, cooler air moves in to take its place and absorb more of your warmth.
There are two types of convection: natural convection and forced convection. Natural convection is produced by density differences in a fluid due to temperature differences (e.g., as in “hot air rises”). Global atmospheric circulation and local weather phenomena (including wind) are due to convective heat transfer.
When the warm water flows through these pipes, it is cooled down by the fans. These fans are also present in the pipes. Once the water cools down, it flows back into the engine; hence, obeying the very principle of convection and cooling the engine down.
Convection is a heat transfer mechanism where heat moves from one place to another through fluid currents. Forced convection is a special type of heat transfer in which fluids are forced to move, in order to increase the heat transfer. This forcing can be done with a ceiling fan, a pump, suction device, or other.
Natural convection, known also as free convection is a mechanism, or type of mass and heat transport, in which the fluid motion is generated only by density differences in the fluid occurring due to temperature gradients, not by any external source (like a pump, fan, suction device, etc.).
The convection caused by winds is natural convection for the earth, but it is forced convection for bodies subjected to the winds since for the body it makes no difference whether the air motion is caused by a fan or by the winds.
Natural convection, known also as free convection is a mechanism, or type of mass and heat transport, in which the fluid motion is generated only by density differences in the fluid occurring due to temperature gradients, not by any external source (like a pump, fan, suction device, etc.).
Convection is the mechanism of heat transfer through a fluid in the presence of bulk fluid motion. Whereas in forced convection, the fluid is forced to flow over a surface or in a tube by external means such as a pump or fan.
Convection. Convection is the second mode of heat transfer and is defined as the transfer of heat through the movement of fluids. In the HVAC and Refrigeration field, convective heat transfer can be found in heating and air conditioning systems, whenever a moving fluid passes over a surface at a different temperature.
