Explain the meaning of the following dimensionless numbers respectively. Nu, Re, Pr, Gr Solution Nusselt number (Nu) is a non dimensional heat transfer coefficient. It is a measure of heat transfer rate comparison between the conduction and convection. Nu = h*L / kf It is the ratio of convection to pure conduction heat transfer, where kf is the conductivity of the fluid. Reynolds number (Re) says, whether the flow is inertial or viscous force dominant that leads to draw characteristics of fluid whether the flow is laminar or turbulent. Re = v*L / kinematic viscosity It is the ratio of the inertia and viscous forces. Viscous force provides the dampening effect for disturbances in the fluid. If dampening is laminar flow --> strong enough Otherwise, instability è turbulent flow --> critical Reynolds number Prandtl (Pr) number says about the kind of fluid. It can give the information about the thickness of thermal and hydrodynamic boundary layer. Pr = kinematic viscosity/thermal diffusivity It is the ratio of the momentum and thermal diffusivities. Grashof number is the ratio of buoyancy force to viscous force and plays nearly the same role as the Reynolds number in forced convection. Gr = buoyancy force / viscous force For forced convection, the heat transfer correlation can be expressed as Nu=f (Re, Pr).

1. Explain the mechanisms of internal loading of phosphorus in lakes
Solution
Intoduction:
Lake sediments play an important role in the phosphorus metabolism in lakes. The impact
depends on the tendencies to retain and to release phosphorus. The internal loading will often
determine the eutrophication status of the lake and the time lag for recovery after reduction of the
external loading. Internal loading is most important during the summer. The potential source of
phosphorus in the surficial sediments is very large in comparison to the pools in the water
column. This means that even if only a very small amount is released, it will have significant
impact on the phosphorus concentration in the lake water.The distribution of phosphorus forms
in sediments have been investigated since the 1950s. Generally, vertical profiles of sediment
phosphorus content, expressed on dry weight basis, show an increasing total content towards the
sediment surface. The vertical profile continues up into the water when looking at the particulate
matter.A number of environmental factors are important in the mobilisation processes. Most
studies indicate that sediment bacteria have a significant role in uptake, storage and release of
phosphorus including anaerobic release of iron-bound phosphorus. Several phytoplankton
species have resting stages overwintering on the sediment. When growth is induced, they leave
their habitat in order to shift their life-form to a pelagic one. In Lake Erken the cyanobacteria
Gloeotrichia echinulata has been shown to contribute significantly to the internal loading of
phosphorus during the summer.
Explination :
Lake sediments play an important role in phosphorops metabolism in lakes. The impact depends
on, Firstly, the tendency to retain phosphorous and thereby decrease the availablity in the water
and, Seondly on the ability to release phosphorous when environmental condition changes. The
composition, pre-history and inding capacity of the sediment govern its reactions. The general
view until the 1970's was the sediments acted as sinks for phosphorous with the exception of
peroids of anoxia whn iron bound phosphate is released to the hypo lymnion of stratified
lakes(enisele,1936;mortimer,1941,1942). During the last decademore knowledge has ben
gathered (Bostrom et al., 1982) and a much more complicated picture has been developed
emphasizing the important role of internal loading for lake water quality and especially summer
blooms of phytoplankton in lakes.
Inorganic and organic particle are almost continuously deposited on lake bottos. Allochtonous
particles are to a significant amount minerogenic and more or less inert. Autochtonous plankton

2. and detritus deliver organic material with a high phosphpurous content to the sediment
Precipitation events in the lake water will also contribute to inorganic sediment as do the shells
of diatoms. Due to various mechanical, chemical and biological processes the phosphorus from
these particles can be released into the water again. This release is generally called internal
loading to be seperated from external loading. In hypertrophic lakes, the role the role of the
sediments come into focus after a reduction of the external loading. The internal loading will
then determine the eutrophication status of the lake.and the time lag for recovery. the process is
mostt important during the summer when the external loading generally is at its minimum in
temperate lakes. In the summer it not unusual for the phosphorous concentration in the water of
eutrophic lakes to be several times higher than in the inflowing water.
The potential phosphorous source in the surficial sediments is very large in comparision to the
pools in the water coloumn. This means that even if only a very small amount is released, it will
have significant effect on the phosphorous concentration in the lake water. Phosphorous can
leave the sediment in a dissolved state mainly as a phosphate or as particles. In the latter case,
thetere are two major case, there are two major ways: as resuspended sediment particles or by
migration of resting stages of phytoplankton, To leave the to leave the sediment in the form of a
dissolved state, phosphorus must be mobilised by physical, chemical or biological processes and
then transported to the lake water.