CFD Analysis of Solid Desiccant Dehumidifier Wheel

Desiccant cooling and dehumidification systems control both the air humidity as well as the operating cost by reducing the energy requirements of the supply air systems. This study used flow simulation CFD high resolution to better understand the vapor flow through complex porous media. The CFD simulation of the adsorption cooling system showed that the design could have beneficial effects on the performance of the system. The emphasis is on optimizing the process to remove the moisture, and the optimal process inflow velocity for the particular desiccant wheel model is determined to be between 1.5


I. INTRODUCTION
The desiccant dehumidifier wheel is the crucial alternative for conventional components used in the HVAC system. The dehumidifier wheel is a vital and pivotal component that may be utilized to make large energy savings in building heating, ventilation, and air conditioning systems and to utilize renewable energy. The optimization of air handling systems based on drying wheels is more sophisticated rather than traditional devices, and appropriate modeling tools are required [1].
In human existence and in the time to come, air condition plays a key part. We may thus claim that high-quality energy is needed. However, high-quality energy production requires significant prices and pollutes as raw resources through the usage of coal and gas. Conventional climate control is built on a vapor compression mechanism.
The usage of CFC and HCFCs is hazardous to our environment. This is why research into alternate air conditioning technologies has been undertaken. The desiccant cooling system is one of the greatest solutions for air conditioning since it is powered by cheap energy and low energy levels, such as solar energy, are widely available. The desiccant wheel is an essential aspect of the desiccant cooling system so that the working parameters of the desiccant wheel are optimized [2], [3].
The actual mass of water vapor in 1 kg of dry air is a specific humidity or humidity content. While relative moisture means the ratio between the actual moisture mass in the air at a certain temperature and the maximum humidity that air can maintain at a certain temperature.
There are two criteria used to describe the level of personal comfort with relative humidity and dry-bulb temperature.
People often concentrate primarily on temperature but rarely on moisture, however, the high relative humidity of indoor air can have major health effects on inhabitants.
During the suction process around the air with a relative humidity of about 100 percent cannot absorb the human body's latent heat. Under these settings, sustained durations might lead to people feeling thermally uncomfortable due to increased body temperature. This can lead to breathing and skin issues with citrus effects. In addition, a humid atmosphere that promotes bacterial growth can also be created. Therefore, to maintain personal comfort in a limited place, the degree of humidity in the air must be controlled [4].
India is a tropical land with high daytime temperatures of 29-34°C and relative humidity of 70%-90% throughout the year. India is a tropical country. For the interior environment, the temperature and relative humidity of ASHRAE Standard 55 are suggested at between 23-26°C and between 30-60%, respectively. Climate systems in India are frequently utilized to satisfy the ASHRAE standard. The number of climate control systems used rose from 13,000 in the 1970s to more than 250,000 in the year 1991, to over 1.5 million by 2020. The number was predicted to increase in the future. The increased need for air conditioning, however, has resulted in huge electricity usage. Air conditioning also fulfills the crucial role of moisture management, apart from sensitive cooling. The cooling process and air dehumidification normally are powered by a cooling spool in traditional air conditioning machines. India's high humidity leads to a somewhat high dehumidification burden. As a result of the overcooling process, the conventional approach requires a huge quantity of energy to reduce humidity. Recently, modern air conditioning incorporated independent dehumidification load handling and sensible cooling capacities, which lower its power demand. It is generally integrated with the air conditioning system for the comfort of residential buildings and workplaces requiring around 60%-70% humidity and hospital operating rooms, which require around 50-60% humidity [5].
Some sectors are sensitive to moisture, such as textiles, food, pharmaceuticals, and battery manufacture. To preserve the quality of their goods and machinery, these sectors demand a low humidity environment of 20-55 percent. Humid air will produce metal corrosion, worsened hygroscope material features, and raise micro-organisms' hazardous activities in items. The system contains two

II. DESICCANT WHEEL
A desiccant wheel is a relatively low rotation speed airto-air heater and mass exchanger. The wheel features a desiccant film frame on the same layer. The channels in the frame come in several forms including sinusoidal, wobble, triangle, etc. In the picture, Two primary streams traverse the wheel axially. The porous drying medium for adsorption and desorption of the matrix is cylindrically operated. During the process, the air is dehumidified and the wheel is humidified in the reactivation section of the air. The wheel revolution causes the adsorption portion to reactivate periodically [9], [10].

A. Working of the wheel
While the wheel is rotating, it is also traversed by two different airflows in opposite directions: these are the process airflow and the regeneration one, as shown in  Regeneration air flows through a usually smaller (or at most equal) area, called the regeneration area. Before the regeneration air passes through the wheel, it is heated up.
During the regeneration process, water contained in the wheel is extracted from the desiccant by the airflow, and the desiccant is regenerated. The wheel rotation brings the desiccant material alternatively in the process area and the regeneration one. Passing through the regeneration area, the desiccant material is brought back to the condition it had when last entering the process area, and the adsorption/desorption cycle can start again [12], [13].
The adsorption capacity is thus intuitively foreseen to be a function of desiccant material, the angular speed of the wheel, process, and regeneration areas ratio, geometry of the wheel, and of course temperature, humidity, and velocity of the airflows. Therefore, the choice of the desiccant material plays a crucial role in the design of the wheel and significantly affects the performance of the whole air conditioning system.

B. Desiccant material
Almost all materials can adsorb and hold water vapor.
There are however some, of the so-called desiccant materials, in which said capability is particularly relevant; among these are e.g. activated carbon, activated alumina, silica gel, lithium chloride, and calcium chloride. The most commonly used adsorbent for desiccant wheels is silica gel, i.e., a porous, amorphous form of silica (SiO2). Silica gel has a great affinity for water vapor due to the enormous quantity of microscopic pores: the internal surface area of pores is several orders of magnitude larger than the outer surface area of the adsorbent (Figure 2) [14].

C. Rotating speed
Another element that affects the wheel operation is the rotating speed. At the same process and regeneration air inlet conditions, process air outlet humidity depends on the wheel velocities shown in Figure 3.
First, it is clear that at low speed the desiccant material remains for a long time in the process area, and its dehumidification capacity is exhausted before the material comes into contact with the regeneration air. Increasing the rotating speed, the adsorption capacity is better exploited, and water content in outlet process air reaches a minimum.
If, however, we continue to further increase the speed, desiccant material comes to not use all its adsorption capacity because the time spent in the process area is too short concerning the adsorption time constant. In such a case, the process is dominated by heat transfer and the dehumidification rate decreases. Figure 3 shows an example of such behavior. Note that there is an optimal rotating speed for dehumidification, corresponding to the minimum water content in the outlet process air. For different operating conditions the optimal wheel velocity can be found and exploited to maximize the wheel efficiency [15].   Table 1 shows parameters and parametric variations of desiccant wheel simulation. For this project work, the results were obtained after verifying the following criteria.  Regeneration to process airflow ratio (R/P) 1

A. Preliminary assumptions
As said at the beginning of this chapter, the proposed model deliberately does not take into account some phenomena which would make it too complex for our purposes. Thus, before introducing the governing equations for the single-channel, it is required to underline the preliminary assumptions based on the model construction.
These assumptions are listed below.
• Axial heat conduction and water vapor diffusion in the air are neglected; • all channels are identical, with constant heat and mass transfer surface areas, adiabatic, and impermeable; • the matrix thermal and moisture properties are constant, as are the mass and heat transfer coefficients, and the adsorption heat per unit mass of adsorbed water; • mixing between the process and the regeneration airflows hour. For the following process intake rate 1.5, 2, 2.5, 3, 3.5, 4, 4, 5, 5, 5, 5, 5, 6, 7, 8, 9, 10, 11, 12 m/s about after rotational rate 10, 20, 30 rph, the impact of the modification on the process outlet air temperature was investigated (shown in Figure 4-7).
The output temperature decreases owing to a lower moisture removal in the process region, with an increase in air intake speed. At varying rotational speeds, the difference between the process output temperature is lower at low velocity than the high velocity. In comparison to the method that eliminated moisture, however, the difference remains considerable. As lower wheel speed is almost the same humidity, but at a lower exit temperature, lower wheel speed is preferable. In connection with the following rotational speed 10, 20 and 30 rph, the influence of the change of humidity in outlet reactivation was tested in