This protocol provides a method for drying performance analysis based on surface temperature distribution instead of single-point temperature, which is the traditional way. As part of this, we developed a technique to obtain even moisture distribution in the fabric for investigating the drying performance of fabric and the air impingement. This method could provide insight into the drying of textiles in the heat inciting process, and it could be applied in the paper drying industry.
To begin, start heating an oven to 120 degrees Celsius. Let it dry at that temperature for 30 minutes. Meanwhile, ensure that a hot air blower is connected to an air nozzle.
Over the course of about 30 seconds, slowly move the air nozzle to between 60 and 90 degrees with respect to the sample position. Next, turn on the air blower fan, then supply power to the resistance wire of the heating element. Adjust the current until the desired air temperature is achieved.
The nozzle and the air may be very hot. Don't touch the nozzle, and avoid directing the hot air blower directly on your body during the experiment. Next, hold an anemometer probe perpendicular to the air nozzle, and measure the air velocity.
Adjust the rotation speed of the fan to reach the desired velocity. After that, cover the air nozzle with the heat insulation cover to shield the sample area during setup. Then, mount an infrared thermographic camera about a meter above the air nozzle.
Connect the thermographic camera to a computer and open the instrument software. Then use a needle plate fixture to mount the standard fabric sample 30 millimeters above the air nozzle outlet. Focus the camera on the fabric.
Then, set the temperature unit to degrees Celsius, the thermal radiance to 0.95, the ambient relative humidity to 50%the ambient temperature to 25 degrees Celsius, and the distance between the measured object and the camera to 1.5 meters. Next, confirm that the uniform padder system is connected to an air compressor. Start the compressor, and set its maximum output to 0.8 megapascals.
Manually adjust the pressure regulators for the clamp cylinders to ensure that the pressure will be equal on both sides of the upper roller. Place a water-saturated, pre-weighed fabric sample on the upper roller and run the uniform padder. Turn off the padder afterwards, and confirm that the moisture content is even throughout the sample.
Lastly, once the oven is dry, set it to the sample drying temperature. To begin preparing the fabric sample, cut a 400-millimeter-by-280-millimeter fabric square, and dry it in the oven for about three hours. Weigh the dry fabric, and then soak it in water for five minutes.
Then turn on the padder and set the desired initial pressure. Lay the saturated fabric on the top roller of the uniform padder. Once the sample passes through the rollers, retrieve the sample and turn off the padder.
Weigh the wet fabric sample and calculate the moisture content. If needed, dry the rollers with a paper towel, and run the fabric through the uniform padder again until the desired moisture content is achieved. Then confirm that the air blower temperature and velocity are set correctly, and that the nozzle is covered with the thermally-insulating board.
Mount the sample in the needle plate fixture and start recording the sample temperature with the thermographic camera. Remove the heat-insulating cover so that the hot air impinges on the underside of the sample. Monitor the increasing fabric temperature on the computer.
Once the temperature is approximately stable for at least 30 seconds, the fabric has dried to the target level. Stop recording the temperature, cover the nozzle with the heat-insulating cover, and remove the sample from the fixture. Set the target area in the video for analysis, and extract the temperature data.
Navigate through the video, and save individual frames from notable points in time as pseudocolor images. Convert the saved frames to grayscale, and identify the intensity value for the fabric that is close to the hot air temperature. Binarize the grayscale images using that value as the threshold to display the dry regions.
This cotton fabric showed a clear temperature decay from the center of the periphery under air-impingement drying. The center of the drying fabric reached a steady-state temperature after 20 seconds of heating. Minimal heating was observed at the edges of the impingement area.
Increasing the temperature of the impinging air sped up the time to dryness, as did increasing the air velocity. The fabric thickness also significantly affected the drying time. Air impingement angles under 75 degrees substantially decreased the temperature at the drying center, and increased the time to dryness.
Air impingement angles under 75 degrees also correspond to smaller impingement areas. Binarization analysis showed that a 70-degree impingement angle reduced the drying area by about 20%and a 65-degree angle reduced the area by about 40%relative to a 90-degree angle. The keys to obtaining the accurate drying performance of the seen fabric is the even distribution of the initial moisture content and the contiguous measurement of surface temperature.
The research paves the way for future studies of the drying characteristics of sheet materials, such as fabric and paper.
