Hello
I am measuring the vaporisation of a water pool in a given climate.
Is there a way to use Wufi to simulate it?
In this case, how to describe the properties of water in the software?
Thank you and best regards
Vaporisation of a water pool
Hi Michel,
WUFI is not really intended to be used for this kind of investigation, but it may be possible to simulate some of the relevant factors.
WUFI does not treat water as a separate material. The water content is one of the properties of the materials making up the building component, and WUFI investigates how this property changes in dependence of the climatic conditions etc.
So you cannot have a 'layer of water', but you can define a kind of sponge-like material with very high porosity which can simulate some aspects of a layer of water.
The evaporation from the pool surface will depend on the vapor pressure at the surface, which is always equal to the saturation vapor pressure corresponding to the given temperature. So it is important to make sure that the vapor pressure at the surface of the simulated water layer, too, is always the saturation vapor pressure, i.e. the relative humidity there must always be 100%.
The relative humidity in turn is determined by the moisture storage function of the material. Moisture contents between free saturation wf and maximum saturation wmax correspond to a relative humidity of 100% (that's what you want), moisture contents below free saturation correspond to lower relative humidity (that's not what you want).
So in order to simulate the constant saturation conditions at the surface of the pool you can fill your sponge material with an initial water content well above free saturation. The relative humidity at the surface will remain at 100% as long as the water content remains somewhere between free and maximum saturation. (Actually, WUFI varies the relative humidity from 100% at wf to a fictitious 101% at wmax because it needs a unique relationship between relative humidity and moisture content. Under ordinary circumstances this little peculiarity is completely negligible.)
In order to stay within the range of constant relative humidity, you should choose a high wmax (say, 990 kg/m3) by means of a high porosity (e.g. 0.99) and a low wf (say, 50 kg/m3) by means of an appropriate moisture storage function.
Of course, any evaporative losses will change the density of the water in this model material; there is no water level that could be lowered. In order to avoid a density gradient in the 'water', high liquid transport coefficients (say, 1e-6) and a low mu value (e.g 1) should avoid major gradients and keep the water density homogeneous.
Using rain water to replenish the pool will not work if the sponge material is supersaturated (above free saturation), so enough water content has to be provided at the beginning of the simulation to avoid drying-out of the sponge.
Now, having implemented the 'water', what can WUFI do?
The amount of evaporation will mainly depend on the humidity of the ambient air, the vapor transfer resistance at the pool surface and the vapor pressure at the surface, i.e. the surface temperature.
The humidity of the ambient air is read from a weather file and it is up to you to provide suitable humidity data. The inverse influence (of the pool on the ambient conditions) cannot be accounted for.
The vapor transfer resistance at the surface is estimated from the heat transfer resistance (the vapor transfer coefficient is assumed to be 7*10^-9 times the convective part of the heat transfer coefficient). The heat transfer coefficient, in turn, is assumed to be constant or, alternatively, estimated from the wind speed via some formula. If your investigations involve determining the dependence of the vapor transfer resistance on various ambient influences, the WUFI model is probably too simplistic for your purposes. Here WUFI does nothing which you could not do with pencil and paper, too.
The vapor pressure at the pool surface depends on the water temperature, and modeling this water temperature might be a point where WUFI's thermal model may be useful to you. It would be possible to build a simple thermal model of the pool:
WUFI takes the evaporative cooling into account.
The heat exchange with the ground can be accounted for as long as the pool and the ground can be treated as infinite layers resting on top of each other.
The heating of the water and the bottom of the pool by solar radiation can be taken into account by appropriate heat sources, but the variability of the reflectivity of the water surface due to different angles of irradiation cannot be modeled. Likewise, the exchange of long-wave radiation between the pool surface and the surroundings can be taken into account if it is not too complex.
The effect of water convection on the temperature profile in the pool can not be modeled; if necessary, you may be able to allow for it by using a suitable effective thermal conductivity of the water.
WUFI automatically adds the thermal capacity of the water to the thermal capacity of the dry material. A very low capacity should therefore be used for the 'sponge' material; a certain variation of the thermal capacity of the 'sponge + water' material with changing water content cannot be avoided.
So it's up to you to decide to which extent WUFI can be useful for your purposes. Could you give us a reference to your investigations once they are finished? We might be interested in your results to improve our models...
Kind regards,
Thomas
WUFI is not really intended to be used for this kind of investigation, but it may be possible to simulate some of the relevant factors.
WUFI does not treat water as a separate material. The water content is one of the properties of the materials making up the building component, and WUFI investigates how this property changes in dependence of the climatic conditions etc.
So you cannot have a 'layer of water', but you can define a kind of sponge-like material with very high porosity which can simulate some aspects of a layer of water.
The evaporation from the pool surface will depend on the vapor pressure at the surface, which is always equal to the saturation vapor pressure corresponding to the given temperature. So it is important to make sure that the vapor pressure at the surface of the simulated water layer, too, is always the saturation vapor pressure, i.e. the relative humidity there must always be 100%.
The relative humidity in turn is determined by the moisture storage function of the material. Moisture contents between free saturation wf and maximum saturation wmax correspond to a relative humidity of 100% (that's what you want), moisture contents below free saturation correspond to lower relative humidity (that's not what you want).
So in order to simulate the constant saturation conditions at the surface of the pool you can fill your sponge material with an initial water content well above free saturation. The relative humidity at the surface will remain at 100% as long as the water content remains somewhere between free and maximum saturation. (Actually, WUFI varies the relative humidity from 100% at wf to a fictitious 101% at wmax because it needs a unique relationship between relative humidity and moisture content. Under ordinary circumstances this little peculiarity is completely negligible.)
In order to stay within the range of constant relative humidity, you should choose a high wmax (say, 990 kg/m3) by means of a high porosity (e.g. 0.99) and a low wf (say, 50 kg/m3) by means of an appropriate moisture storage function.
Of course, any evaporative losses will change the density of the water in this model material; there is no water level that could be lowered. In order to avoid a density gradient in the 'water', high liquid transport coefficients (say, 1e-6) and a low mu value (e.g 1) should avoid major gradients and keep the water density homogeneous.
Using rain water to replenish the pool will not work if the sponge material is supersaturated (above free saturation), so enough water content has to be provided at the beginning of the simulation to avoid drying-out of the sponge.
Now, having implemented the 'water', what can WUFI do?
The amount of evaporation will mainly depend on the humidity of the ambient air, the vapor transfer resistance at the pool surface and the vapor pressure at the surface, i.e. the surface temperature.
The humidity of the ambient air is read from a weather file and it is up to you to provide suitable humidity data. The inverse influence (of the pool on the ambient conditions) cannot be accounted for.
The vapor transfer resistance at the surface is estimated from the heat transfer resistance (the vapor transfer coefficient is assumed to be 7*10^-9 times the convective part of the heat transfer coefficient). The heat transfer coefficient, in turn, is assumed to be constant or, alternatively, estimated from the wind speed via some formula. If your investigations involve determining the dependence of the vapor transfer resistance on various ambient influences, the WUFI model is probably too simplistic for your purposes. Here WUFI does nothing which you could not do with pencil and paper, too.
The vapor pressure at the pool surface depends on the water temperature, and modeling this water temperature might be a point where WUFI's thermal model may be useful to you. It would be possible to build a simple thermal model of the pool:
WUFI takes the evaporative cooling into account.
The heat exchange with the ground can be accounted for as long as the pool and the ground can be treated as infinite layers resting on top of each other.
The heating of the water and the bottom of the pool by solar radiation can be taken into account by appropriate heat sources, but the variability of the reflectivity of the water surface due to different angles of irradiation cannot be modeled. Likewise, the exchange of long-wave radiation between the pool surface and the surroundings can be taken into account if it is not too complex.
The effect of water convection on the temperature profile in the pool can not be modeled; if necessary, you may be able to allow for it by using a suitable effective thermal conductivity of the water.
WUFI automatically adds the thermal capacity of the water to the thermal capacity of the dry material. A very low capacity should therefore be used for the 'sponge' material; a certain variation of the thermal capacity of the 'sponge + water' material with changing water content cannot be avoided.
So it's up to you to decide to which extent WUFI can be useful for your purposes. Could you give us a reference to your investigations once they are finished? We might be interested in your results to improve our models...
Kind regards,
Thomas