Study on the effect of pressure in humid air dehumidificationournal

In this paper, there are problems such as large energy consumption and complex system in the field of condensation dehumidification. It is found that increasing pressure can strengthen the effect of condensation dehumidification, simplify the system and reduce energy consumption. So this paper simulates the condensation and dehumidification process of humid air in horizontal pipe under positive pressure. The results show that the dehumidification capacity increases by 25%∼30% when the pressure increases by 100kPa on average. The increase of pressure has little effect on the heat transfer coefficient, but the increase of humidity increases the heat transfer coefficient more obvious. When the humidity of wet air increases by 20%, the heat transfer coefficient increases by 18% ∼ 33%. The dehumidification capacity and heat transfer coefficient increase with the increase of pipe diameter and fluid flow rate.


Introduction
In the context of a low-carbon economy, reducing carbon emissions has become a common goal for countries worldwide.High-efficiency condensation dehumidification technology can help reduce energy consumption and carbon emissions, making it increasingly important [1].By using efficient condensation dehumidification technology to dehumidify humid air, it can increase the efficiency and performance of air conditioning and refrigeration systems while reducing energy waste and carbon emissions.Therefore, the active use of efficient condensation dehumidification technology to treat humid air is needed for future development to promote the development of a low-carbon economy while ensuring the comfort of people's lives and the health of the environment [2].To improve the efficiency of condensation dehumidification technology for humid air as the dehumidification object, scholars have conducted extensive research.When the humid air condenses on the wall, there is a thin film of condensed water on the wall, which makes the heat transfer efficiency low.But for moist air condensation dehumidification, improving the surface heat transfer efficiency is the key to improve the heat transfer efficiency.Ren [3]and others found that the subcooling in the condensate film can reach 19.78°C, and at this time, the thermal resistance in the condensate film is high.Scholars have conducted in-depth research on the fluid flow pattern in order to obtain corresponding results.
Based on the existing problems in the field of condensation dehumidification, such as low dew point temperature requirement, the need for chiller units in the system, and high energy consumption, this study deeply investigates the issue of condensation dehumidification [4].It is found that current condensation dehumidification is mostly based on atmospheric conditions, and pressure is an important parameter for condensation dehumidification.Increasing pressure can raise the dew point temperature of vapor in humid air, which make the condensation process more efficient and rapid, thereby simplifying the system and reducing energy consumption.However, no scholars have conducted specialized research on positive-pressure condensation dehumidification [5], so this study uses the VOF to simulate condensation process of moist air in tube channels, further studying the moist air and providing a certain theoretical basis and scientific guidance for strengthening heat transfer of moist air.

Physical model
Phase transitions are involved between the wet air and the cold wall surface, and the two phenomena occur simultaneously and influence each other.During the transition from gas to liquid water, the physical properties of vapor change significantly, and simulating this complex process is challenging and requires simplification of the actual process.The hypothesis of this study is: (1) The moist air is an incompressible ideal gas, and the mixture is a single-phase multi-component fluid composed of air and vapor; (2) Since the temperature of moist air changes little during the entire condensation process, and the volume fraction of vapor in moist air is small, physical properties of moist air during the condensation process are considered unchanged; (3) The pressure and velocity in the circular tube are assumed to be constant.
The humid air flow model is shown in the Figure 1, where the tube wall is made of aluminum, the tube diameter is D = 10 mm, and the tube length is L = 500 mm.The calculation area is the area between the inner surface of the tube and the humid air flow.In order to save memory, reduce calculation time and ensure the accuracy of calculation, the three-dimensional model is transformed into a two-dimensional model.The simulation conditions are as follows: gravity acceleration is 9.81m/s2, pipe diameter is 10mm, wall temperature is 283K, inlet temperature is 300K, and moist air velocity is 3m/s.The numerical simulation results of condensation on the air side under humid operating conditions of the condensation heat exchanger are presented in detail, and the effects of inlet humid air pressure and relative humidity, on condensation characteristics on the air side are analyzed.The pressure change of the model is from 101.325kPa to 1100kPa, the selected relative humidity is RH=60-100%, and the selected moist air velocity is 3m/s.

Mathematical model
In this paper, the humid air is treated as an ideal gas and the condensate of vapor is assumed to be quickly removed.The condensation rate formula at the fluid-solid interface considering the combined effects of condensation.
Where, D is the mass diffusion, unit m/s 2 , for humid air: Where, P0 is the standard atmospheric pressure in Pa.When the temperature of the humid air does not reach the dew point temperature under this pressure, the vapor in the humid air cannot condense, and there is no liquid water precipitation near the wall.When the temperature of the humid air reaches the dew point temperature under this pressure, the vapor in the humid air gradually begins to condense, and there is liquid water precipitation near the wall.
Mass and component source terms:

Results and discussion
The simulation conditions are as follows: gravity acceleration is 9.81m/s2, pipe diameter is 10mm, wall temperature is 283K, inlet temperature is 300K, and humid air velocity is 3m/s.The numerical simulation results of condensation on the air side under humid operating conditions of the condensation heat exchanger are presented in detail, and the effects of inlet humid air pressure and relative humidity, on condensation characteristics on the air side are analyzed.The pressure change of the model is from 101.325kPa to 1100kPa, the selected relative humidity is RH=60-100%, and the selected moist air velocity is 3m/s.

Cloud diagram of vapor mass fraction in humid air
When the pressure is 160kPa, the inlet velocity is 3m/s, and the relative humidity is 60%, 80%, and 100%, the steam mass fraction at different relative humidity of the circular tube passage is shown in Figure 2(a).It can be observed that the mass fraction of the steam varies greatly in the first half of the tube due to condensation of the steam.At the back of each tube there is a wake zone.The vapor mass fraction is low and the air mass fraction is high in the wake area, so the condensation is weak.
Figure 2(b) shows the cloud diagram of vapor mass distribution in humid air at different inlet velocities when the inlet relative humidity is 80%.The wake area of each pipeline gradually disappears with the increase of speed, so under the same relative humidity at the inlet, the vapor mass distribution in humid air at the outlet of the round pipe channel increases with the increase of the inlet speed.The Figure 3 shows the distribution of vapor mass fraction in humid air at a distance of 0.125m from the pipe entrance.x indicates the distance from the center of the tube to the wall and wa represents the vapor mass fraction in the humid air.As can be seen from the figure, the closer to the tube wall, the lower the mass fraction of vapor in the humid air, which is caused by the continuous condensation of vapor in the humid air on the tube wall.

Distribution cloud diagram of humid air temperature distribution
When the pressure is 160kPa, the inlet velocity is 3m/s, and the relative humidity is 60%, 80%, and 100%, the temperature distribution at different inlet relative humidity is shown in Figure 4(a).The temperature distribution of humid air in the pipe is basically the same, and the relative humidity has little effect on the temperature distribution of wet air.

Effect of pressure on moisture content
It can be seen from the Figure 6 analysis that under the condition of constant relative humidity, the outlet moisture content decreases with the increase of pressure, which is particularly obvious in the early stage of pressurization, because the humid air can carry less vapor in the early stage of pressurization, resulting in a rapid decline in moisture content.When the pressure is constant, the relative humidity has little effect on the outlet moisture content.

Effect of pressure on dehumidification capacity
As can be seen from Figure 7, when the pressure increases, the dehumidification amount also increases significantly.Under the same humidity, the initial dehumidification changes the most with the increase of pressure, and gradually becomes stable.When the pressure reaches 1100kPa or above, the dehumidification amount remains stable under the same relative humidity.

Conclusions
A two-dimensional model was established using CFD software Fluent for numerical simulation and analysis of positive pressure condensation heat transfer of humid air in horizontal tubes under different pressures and relative humidity, and the feasibility was verified through experiments.The changes in surface heat transfer coefficient and dehumidification capacity of the horizontal tube with pressure, relative humidity, flow velocity, and tube diameter were analyzed, and the following regularities were obtained: (1) When the air humidity is constant, increasing the pressure in the pipeline raises the dew point temperature of the air, which results in more rapid condensation and increased heat transfer efficiency.For each increase of 100kPa, the dehumidification capacity increases by about 25% to 30%.When the pipeline pressure is constant, increasing the humidity of the air leads to more vapor saturation in the humid air and thus to more condensate under the same conditions.
(2) The condensing film gradually thickens from the inlet to the middle of the tube, and the film thickness remains constant at the back of the tube.As the amount of condensate increases, the condensate settles at the bottom of the tube due to gravity, causing the liquid film at the bottom of the tube to be thicker than the liquid film at the top.
(3) Pressure does not affect the heat transfer coefficient, which increases with increasing relative humidity.The heat transfer coefficient increases by about 18% to 33% for every 20% increase in humidity of the humid air.

Figure 2 .
Figure 2. Distribution of vapor mass fraction at different relative humidity and inlet velocity.

Figure 3 .
Figure 3. Mass fraction of vapor in humid air.

Figure 4 (Figure 4
Figure 4 Humid air temperature distribution in a circular tube channel at different inlet velocities and relative humidity.

Figure 5
Figure5shows the temperature distribution of humid air in the tube at different speeds.The abscissa xa indicates the distance from the inlet, and the ordinate indicates the temperature of the humid air.Due to the continuous heat transfer in the tube, the humid air temperature is continuously reduced from the inlet to the outlet.The flow rate increases, the heat transfer time is shortened due to the shortening of the residence time of the fluid in the tube, and the outlet temperature of the humid air is also increased.

Figure 5
Figure 5 Temperature distribution of humid air at different speeds.

Figure 6
Figure 6 Change of dehumidification under different pressure and relative humidity.

Figure 7
Figure 7 Changes of outlet air moisture content of humid air under different pressure and relative humidity.