Study on the Moisture Absorption and Desorption Rate of Bamboo Scrimber in a Hot-Humid Climatic Wind Tunnel

Summary

Here, we present a protocol to measure the moisture absorption and desorption rate of bamboo scrimber in an outdoor environment for a dynamic hot-humid climate wind tunnel test with complete meteorological conditions for 72 h.

Abstract

The absorption and desorption rate U_ab is an important indicator to evaluate the hygric performance of hygroscopic building materials. However, standard methods to obtain this value are normally carried out in a static indoor environment, which cannot represent a dynamic outdoor environment. This protocol presents a Hot-Humid Climate Wind Tunnel (HHCWT) method to examine the hygric behavior of bamboo scrimber (BFB) in outdoor conditions, with the hourly solar radiation, air temperature, relative humidity, and constant wind speed of a typical summer day in the south of China. The complete meteorological parameters were repeated for 72 h, of which the last 48 h were selected for analysis. By comparison with the hardwood specimen (HW) in this test, BFB showed a more stable hygric performance. The BFB to HW ratio of the U_ab mean value and maximum value were, respectively, 64.35% and 66.02%. The maximum absorption rate U_a.max and desorption rate U_d.max from the dynamic test were correspondingly 2.91E^-05 kg/(m²/s) and 4.45E^-05 kg/(m²/s), far larger than the results of 7.74E^-07 - 12.58E^-07 kg/(m²/s) from the static test. The significant difference in magnitude shows the necessity of a dynamic evaluation approach that can take more practical climate conditions into consideration. The HHCWT enables the reproducibility and standardization of climate-related experiments for building materials by creating climate conditions with complete meteorological parameters.

Introduction

Bamboo scrimber is considered to be an ideal product to achieve a high value-added utilization of the widely distributed bamboo forest resources, by decomposing raw bamboo and recombining it into bamboo panels that could be applied as outdoor flooring or a building facade. There already are studies on the hygrothermal properties of bamboo scrimber with static standard methods carried out in indoor environments^1,2. However, the performance in more practical outdoor conditions needs further investigation. The hygric properties of bamboo scrimber obtained in our prior study are shown in Table 1.

The moisture absorption and desorption rate U_ad are an important indicator characterizing the hygric transport performance, and also closely affect the thermal performance of certain building materials, especially for those hygroscopic materials such as bamboo and timber. Normally, the U_ad value is measured by static standard methods in a laboratory environment, with constant ambient air temperature T and relative humidity RH (e.g., T = 23 °C, RH = 50%)³. However, the constant indoor testing conditions are very different from the practical application in an outdoor environment, where the meteorological conditions are more complex. The effect of solar radiation on the heat and moisture transfer cannot be ignored^4,5. Other studies have investigated the dynamic performance by establishing dynamic models and transfer functions^6,7 andshow different characteristics compared to the static standard tests⁸.

To test the material objects in real outdoor climate conditions is a more persuasive method to evaluate the targeted performance. However, outdoor climate conditions do not reappear, making the experiments difficult to standardize, let alone when done by different operators at different periods. For solving the above challenges, we studied reproducible meteorological conditions and developed them with a climate wind tunnel. A wind tunnel is a comprehensive concept to create different simulated conditions (e.g., wind resistance testing, evaporative cooling, or a microclimate on building envelope) to examine certain performances of the research objects^9,10,11,12.

For this study, the HHCWT was created with a motivation to reproduce the outdoor climate conditions for examining the hygrothermal performance of building materials and building components closer to the practical application conditions^13,14. The HHCWT consists of 18 elements: a protection room (Figure 1.1) with an air conditioner, a heater, a humidifier, a dehumidifier (Figure 1.12 - 1.15), a wind tunnel (Figure 1.2) with an entrance region, a stilling region, a test region, an auxiliary region, a diffusion region and a fan region in the order (Figure 1.5 - 1.10), a solar radiation chamber (Figure 1.11) overhead and an air conditioning chamber (Figure 1.16) below, the specimen slot (Figure 1.4) with a balance (Figure 1.17) on the boundary of the wind tunnel and the air conditioning chamber (Figure 1.3), and the operating condition control and data collection system (Figure 1.18) outside the protection room. An operating condition control and data collection system were set outside the protection room for the input and output control of the whole system. The climatic wind tunnel enabled the complete control of climate conditions, therefore realizing the repeatability and standardization of the climate-related experiment (Figure 1).

In this study, a dynamic absorption and desorption pair-test was carried out for bamboo scrimber and hardwood specimens. The HHCWT was used to create complete climate conditions, including the solar radiation, air temperature, relative humidity, and wind speed of a TMD (Typical Meteorological Day) of Guangzhou, a typical subtropical city located in the south of China. The obtained values were compared with the results from the static standard method to show the progress of this experimental method.

Since factors such as bulk density, open porosity, and surface treatment had an impact on the hygric properties of the products^15,16, typical samples were selected from the company that owned the core patent (Table 2).

Protocol

1. Specimens Treatment

1. Specimens preparation
   1. Select the material: for the bamboo (BFB), a bamboo scrimber panel made of Moso (Phyllostachys Pubescens); for the timber (HW), a hardwood panel made of Intsia spp. (Caesalpiniaceae).
   2. Ask the bamboo and wood factories to cut the specimens to a size of 30 x 30 cm (side length), while keeping the original thickness of 28 mm.
      Note: The normal size of the bamboo panels used is 122 x 244 cm (width x length), and the thickness is 28 mm or 18 mm. The samples can best be cut at the factories because they have the suitable cutting machines. Chose the thickness of 28 mm instead of 18 mm, because the thicker sample generally ensures more stable results.
   3. Prepare the specimens in duplicate.
2. Vacuum saturation
   1. Arrange the equipment as follows: 1 vacuum chamber and pump (a sliding vane rotary vacuum pump, extreme pressure 600 Pa, speed 8 L/s); 1 independent and movable water sink of 35 x 35 x 20 cm (length x width x height); and 20 stainless-steel nut supports of equal size.
   2. Plain lay the stainless-steel nut supports on the bottom of the water sink, and side stand the specimens on the supports.
   3. Suppress it with a stainless-steel grid on the top.
      Note: The weight of the grid should be enough to ensure that the specimens can be kept immersed in the water when the water is later injected into the water sink.
   4. Move the specimens with the water sink into the vacuum chamber, then close the chamber and make sure that the water inlet valve has already been closed.
      Note: The chamber is part of the vacuum pump, and the size of the chamber is 40 x 40 x 40 cm (length x width x height). If another type of machine is used, it can be a different size, as long as it is large enough to hold the water sink.
   5. Connect the power plug of the vacuum pump to the 380 V electric source.
      Note: The voltage of the power supply is decided by the requirement of the vacuum pump. We needed 380 V for the device we used, but it may also be 220 V or other values depending on the devices used.
   6. Press the start switch to start the vacuum pump. Lower down the air pressure in the chamber to 20 mbar. Keep the vacuum pump running to hold the air pressure inside the chamber ≤ 20 mbar for 6 h.
      Note: The purpose of this step is to make sure that the air inside the internal pores of the specimens is exhausted, so that water can fill in the pores in the following steps. Different materials might require different time lengths due to a difference in the transport resistance of the materials.
   7. Keep the air pressure in the vacuum chamber ≤ 20 mbar, as mentioned above. Turn on the water inlet valve to slowly inject distilled water into the sink.
   8. Keep the water level rise rate to be about 5 cm/h. Stop injecting water when the water level reaches about 5 cm over the top surface of the specimens, then turn off the vacuum pump.
   9. Keep the specimens immersed in water for 3 days, until an initial moisture content of capillary saturation is reached.
   10. One by one, take specimens out of the water and wipe off the attached water from the surface with a soft damp cloth.
       Note: Due to the water surface tension, some water attached to the surface of the specimens will follow up when the specimens are taken out of the water.
3. Bottom and side sealing
   1. Air-dry the specimens in an indoor environment (ca. T = 23 °C, RH = 50%) for 1 h, until the surface is dry.
      Note: Make sure that the specimens are not covered by anything that might stop the surface from drying.
   2. Seal the bottom and side surfaces with 1 - 2 mm petroleum jelly.

2. Wind Tunnel Operation

1. Equipment arrangement
   1. Check the system components, as shown in Figure 1 and Table 3.
   2. Reset the computer time of the operating condition control and data collection system according to the climate data.
      Note: In this step, each computer has different requirements. Click on the time in the lower right corner and input the condition time. It might be different at another computer system. In the following introduction, all the mentioned time is condition time that is synchronized with the climate data. The preheating of the wind tunnel is set to be at 4:00, the specimens’ installation at 5:00, and the data recording starts at 6:00. The 24 h single-days are required to be repeated continuously at least 3x.
2. Wind tunnel preheating (at 4:00)
   1. Start the wind tunnel system for preheating. Run it with the climate parameters as shown in Table 2.
      Note: The TMD in this study is from Guangzhou and can be changed to other cities according to the experimental purposes (Table 4).
   2. Observe the received data from the wind tunnel and ensure the hourly deviations of the real climate conditions in the test region are within the scope of the error requirements: the air temperature ≤ 0.3 °C, the relative humidity ≤ 5%, the solar radiation ≤ 5%, and the wind speed ≤ 5%. Extend the preheating time, if necessary.
      Note: The scope of the error requirements is confined to the accuracy of the sub-systems of the HHCWT. A smaller or larger range might be also acceptable if the equipment is different.
   3. Keep the air temperature in the air conditioning chamber at 26 °C ± 1 °C, and the wind speed ≤ 0.5m/s.
3. Specimens installation (at 5:00)
   1. Place the specimens into the test tanks and seal the gap between the edges of the specimens and the tanks with petroleum jelly.
   2. Place the specimens in the test region in the test tanks of the wind tunnel and connect the bottom of the tanks to the balance below with prefabricated steel frames.
      Note: The function of the steel frames here is to transfer the gravity of the specimens to the balances below. All specimens are tested together but the mass change rate for each specimen is recorded individually by each balance below.
   3. Adjust the height of the balance to keep the top surface of the specimens’ flush with the inner surface of the wind tunnel.
      Note: In this operation, make sure that the test tanks do not come into contact with each other to avoid mass measurement errors.
4. Data recording (at 6:00)
   1. Reset the balances to start the mass record at 0.
      Note: Press the Clear of the balances to reset them to 0.
   2. Use the computer (the operating condition control and data collection system) to record the data.
      Note: During the data recording, pay attention to the data receiving ports for instantaneous mass values and climate conditions, and also to the alarm (which only happens when the real hourly deviations of the climate conditions run beyond the error tolerance).
   3. Repeat the 24-h climate control for 3x, and continuously record for 72 h.

3. Post-operation

1. Dry the specimens
   1. Arrange the following equipment: a drying oven (a digital stainless steel electric blast oven with an accuracy of ± 1 °C) and a balance (with an accuracy of 0.01 g). Keep the drying temperature of the oven at 70 °C.
   2. Place the wet specimens in the drying oven and weigh 1 week later.
   3. Continue to weigh the specimens each 24 h and measure the convergence until the difference between the two consecutive weighing results does not exceed 1%. Record the final weighing result.

4. Data Processing

1. The values of the mass
   1. Average all the instantaneous mass values within each h as the mass of time i (m_i).
      
      Where:
      m_i is the specimens’ mass of time i in kg;
      i–1,j is the jth measured value within 1 h after time i–1 in kg;
      n is the number of measured value within 1 h (e.g., n = 60 when the recording interval is 1 min).
   2. Consider the final value in step 3.1.3 as the dry mass of the specimens m_d.
2. Calculate the absorption and desorption rate U_ad value.
   
   Where:
   U_ad,i is the absorption and desorption rate of time i (when the U_ad,i > 0, it means absorption rate; when the U_ad,i < 0, it means desorption rate) in kg/(m²/s);
   m_i, m_i–1 is the mass value of time i and the value at the previous time i–1 in kg;
   A is the measurement area of the specimens in m².

Results

The recorded data of the last 48 h from the whole 72 h were selected and analyzed (Figure 2, Table 5).

The dynamic test in the wind tunnel was performed and the results showed that the bamboo scrimber (BFB) had a lower U_ad value than the reference hardwood (HW). Compared to HW, BFB showed a more stable hygric performance. The U_ab mean value U_ab.mean ...

Discussion

The protocol presented a Hot-Humid Climatic Wind Tunnel (HHCWT) test method to measure the moisture absorption and desorption rate of bamboo scrimber. The critical steps included the specimen's treatment, which in turn included the specimen's preparation, vacuum saturation, and bottom and side sealing; the wind tunnel operation, which in turn included the equipment arrangement, wind tunnel preheating, specimens installation, and data recording; the post-operation including drying the specimen; the data processing...

Materials

Name	Company	Catalog Number	Comments
bamboo scrimber sample	Dasso Industrial Group Co. Ltd. Hangzhou, China	-	made of Phyllostachys Pubescens, mainly used for outdoor flooring
Hardwood sample	Shan Shang industry, Shanghai, China	-	made of Intsia.spp, Caesalpiniaceae, mainly used for outdoor flooring
Petroleum jelly	Guangzhou Suixin Chemical Co., Ltd.	-	White Vaseline
Balance	Shimadzu	UX 6200H	Range 0.5-6200 g, accuracy 0.01g
Vacuum pump	Dongguan Fopump Vacuum Equipment Co., Ltd., Dongguan, China	2X-8	Sliding vane rotary vacuum pump, extreme pressure 600Pa, speed 8L/s
HHCWT: Hot-Humid Climatic Wind Tunnel, including control systems for:	the Building Energy Conservation Research Center of South China University of Technology	-	The HHCWT consists of 18 elements (table 3)
Software attached to the HHCWT operating condition control and data collection system	the Building Energy Conservation Research Center of South China University of Technology	-	Created by visual programming language Visual Basic