Frost Protection System | FPS
PRINCIPLES OF ADDITIVE WATER FREEZING TECHNOLOGY TO CREATE THIN-WALLED ICE CRYSTAL STRUCTURES | ANALYSIS AND INFLUENCE OF ICING PARAMETERS ON THE THICKNESS AND TRANSPARENCY OF THE PROTECTIVE ICE STRUCTURE FOR THE PURPOSE OF DEVELOPING FROST PROTECTION SYSTEMS
X.1
Fluid flows along the surface of a flat plate in the x-direction. In this problem assumes plug flow, the velocity is uniform and does not vary with distance normal to the plate surface. The plate has length L. We are interested in the local heat transfer coefficient, h(x), in the back of the plate, from x = L/2 to L. Consider four conditions for the plate temperature distribution and rate them from maximum to minimum h(x):
a) The plate has a uniform temperature Tw over the entire plate length L. The ambient
temperature, T amb is lower than T w.
Adding insulating materials (polyethylene, fiberglass, polystyrene, etc.) to ice protection surfaces is sometimes more effective than irrigating with anti-freeze liquids.
For design purposes, the insulator's effectiveness is expressed as an equivalent concentration of glycol or antifreeze liquid (saturated solution of deicing salts).
Fluid flows over the flowerpot insulator are of the laminar type. The main flow direction is indicated by arrows, and flow separation takes place near the insulator vertex. When water freezes to ice, its latent heat of fusion is released, which keeps the insulator warm for some time after liquid water has stopped flowing on it.
Released heat creates quasi-liquid layers (QLLs) with insulating properties that can protect plants against frost.
For example, a QLL of water may insulate plants below it from the effects of sub-zero air temperature on exposed objects.
Typically, insulators are used to provide protection to exposed objects at risk of damage due to sub-zero air temperature. A wide range of insulators is commonly used including cotton, plastic foams, and straw bales (see Table 1). One disadvantage of insulating with straw is its bulkiness; hence an alternative insulator is desirable for protecting objects with minimal screen area.
Table 1.
This research investigates how QLLs of water form at surfaces exposed to sub-zero air temperatures and the potential benefits this has for frost protection system (FPS)
b) The front half of the plate (x =0 to L/2) has a temperature of 1, 2 Tw while the back half is at Tw,
In this example, pear tree insulators are simulated with the help of insolation data. It is assumed that insulators protect tree trunks from damages caused by ice formation. The insulating material must have a low thermal conductivity to minimize heat losses through convection so insulator materials were chosen as water or glass wool (Insulation rating: R-value = 2.4 mK/W).
The insulator has a plate shape and there is no contact between it and the trunk, insulator surface faces the space available for convective flow; furthermore, the insulating layer consists of its own material which can freeze and melt during the heating and cooling process respectively (see Figure 1).
Figure 1.
It can be assumed that insulators prevent any air movement within them and in this state keep the plant alive.
FPS | OPG Stjepan Car | Peach
c) The front half of the L/2) has a temperature of O,8 T w while the back half is at T x,
The insulator should be able to reduce the temperature of its surroundings. For example, if insulators are put in places where the plant would get direct sunlight or near to heating elements, it can prevent overheating of the plant. When insulators are not necessarily needed, insulators should be removed/removed from that area to provide good air circulation for the plants.
When rain or ice is falling on insulators, insulators may lower the surface temperatures causing ice formation on insulating surfaces. If there is no air movement in insulators when rainfall occurs, this will result in frost forming inside insulators and also on the outer surface of insulating materials blocking their pores. Frost formation may destroy insulation properties of insulation materials by blocking airflow within insulation (see Figure 2).
Figure 2.
d) The plate has a constant wall heat flux over the entire plate length L.
This may reduce the insulating value of fiber insulations by 15% or more. Blocking of airflow may also result in both reduction of vapor flux through the insulating material and therefore, reduction ice is formed on the surface of insulator which results in damaging effect to building materials.
One way to prevent this is with a frost protection system. The purpose of an external frost protection system is to maintain surface temperature above the dew point during cold weather when it would otherwise be below freezing. Water with antifreeze (glycol) in proportion 1:1 or 2:1 can be sprayed onto surfaces in case of low temperatures up till -5°C. Water droplets evaporate due to insulators’ surface radiation to lose heat.
FPS | Agroflora by Gardens | Cherry
Semicrystalline insulators (polyethylene) used in the buildings may be covered by ice or frost when air humidity is high and ambient temperature is low.
Frost formation
One way of avoiding this is an insulating material with a special coating that keeps the insulating material dry. This keeps the insulator dry due to blocking moisture from the outside atmosphere.
Frost protection systems are mainly based on different types of heating elements. They are basically designed for three kinds of insulators: Cellular glass, rock wool, and cork.
These types of insulators are used in steel constructions including walls, roofs, etc. When using these insulations one must not forget that they have storage capabilities for the 100% RH.
FPS | OPG Svemir Kos | Cherry crystal ice
X.2
Consider the case of uniform plate temperature Tw. Now include a momentum boundary layer of equal thickness to the thermal boundary layer. That is, the fluid velocity varies in the z-direction over the boundary layer from the free stream value W inf to zero at the plate surface. How does the heat transfer coefficient, h(x), for this case compare to a case in the previous problem?
As insulators, the insulating elements are also used to protect plants. These insulating elements are made of high-quality insulating materials that can be easily cut into the desired size. The insulating protective film is very flexible and has a resistance to rain and UV radiation, which makes it particularly long-lasting. They have a typical thickness of 0.1 – 0.3 mm and do not absorb moisture or dirt particles as they consist of pure aluminum foil with a plastic coating. The insulator protection film is ideal for children's playgrounds, walkways, roofed garden rooms, carports & garages as well as for protecting trees from frost damage during the winter months. It is good at keeping out water but still allows ice to form.
A new insulator called extra-porous film of ethylene tetrafluoroethylene (ETFE) has been discovered by researchers from the Vienna University of Technology. This material is transparent and consists of a grid, on which the ice crystals form - but they do not grow into big blocks. It is a kind of insulating surface for the air-conditioning in high rise buildings, but it also makes a good insulator against water and ice.
is not inexpensive to produce. "We have already found something cheaper with even better insulating properties," said Professor Günther Leitinger from the Institute of Applied Polymer Research at TU Wien.
X.3
Again, consider the momentum boundary as in X.2. In this case, the front of the plate (x = 0 to L/2) has a temperature of T inf while the back half is at Tw. How does the heat transfer coefficient, h(x), for this case for the second half of the plate compare to h(x) for the front half of the plate in Problem X.2?
X.4
Fluid flows in steady state in the entrance region between two constant temperature parallel plates. One suggestion is to calculate the heat transfer coefficient in the entrance region as if it was external flow over a flat plate. Will this be accurate prediction for h and explain why?
In some cases these grids may be covered in a material that melts when it gets cold, forming a blanket of water over the crystal ice. This is called a frost protection system. Such systems can protect objects from getting damaged by air crystal ices though they are not practical for large areas since they offer little insulation and need maintenance. In addition, crystal ices do not form when there is high humidity in the air because latent heat prevents crystal ice formation. However, if crystal ices are formed, they lower the humidity of the air around crystal ice.
When crystal ices are added to water, crystal ices provide latent heat that must be considered when applying Newton's Law of Cooling to crystal ice droplets. When water is cooled by crystal ice, the temperature of the mixture will always be below 0° Celsius. With regard to steel production, crystal ices can also cause problems because they might damage equipment during transportation and storage before steel production begins. Additionally, crystal ices in certain types of machinery may become wedge-shaped and impede machine function until they melt or are removed. Finally, wedge-shaped crystal ices may freeze freshwater into pipelines when transporting steel products through pipelines.
Application of Newton's law transient cooling
Simple solutions for transient cooling of an object may be obtained when the internal thermal resistance within the object is small in comparison to the resistance to heat transfer away from the object's surface (by external conduction or convection), which is the condition for which the Biot number is less than about 0.1. This condition allows the presumption of a single, approximately uniform temperature inside the body, which varies in time but not with the position. (Otherwise the body would have many different temperatures inside it at any one time.) This single temperature will generally change exponentially as time progresses (see below).The condition of low Biot number leads to the so-called lumped capacitance model. In this model, the internal energy (the amount of thermal energy in the body) is calculated by assuming a constant heat capacity. In that case, the internal energy of the body is a linear function of the body's single internal temperature. The lumped capacitance solution that follows assumes a constant heat transfer coefficient, as would be the case in forced convection. For free convection, the lumped capacitance model can be solved with a heat transfer coefficient that varies with temperature difference.[8]
X.5
An experiment on steady laminar flow through a packed bed of spheres yields a value of Nusselt number where is based on the local heat transfer rate and the difference between the local spheres surface temperature and the local bulk fluid temperature flowing through the space between the spheres. At Reynolds number below 1, the Nusselt number based on the sphere diameter is found to be less than 0.1
A) Does this seem qualitatively correct?
b) If yes, explain why. If not, could you suggest possible explanations for such experimental results?
