Surface wettability significantly affects the condensation and therefore the heat transfer when condensation occurs. The materials are classified as either hydrophobic or hydrophilic. Materials with a lower contact angle are more suitable for heat transfer applications associated with condensation. The dynamic contact angle is one way to define surface wettability. In this contribution, the Wilhelmy method was used for measuring the force of the interaction of a fibre at the liquid-gas interface. The method is based on immersing a sample in a liquid and then removing it. This study compares the influence of the type of polymeric material and surface finishes of hollow fibres on the dynamic contact angle. Polypropylene fibres and polypropylene fibres with a Trapylen ® , (TRAMACO GmbH, Germany) surface finish achieve a larger dynamic contact angle (the advancing dynamic contact angle measured in water is around 100). Therefore, they seem to be a better alternative than polycarbonate, polyether ether ketone and polyamide (the advancing dynamic contact angle in water is around 80).
Aim. The aim of this pilot study was to acquire insight into the parameters of glycaemic control, especially, (1) the time delay (lag phase) between plasma and tissue glucose concentrations in relation to rise and fall in glucose levels and (2) the rate of glucose increase and decrease. Methods. Four healthy people (HP), 4 people with type 1diabetes (DM1) and 4 with type 2 diabetes (DM2) underwent concurrent glucose measurements by means of (1) the continuous glucose monitoring system (CGMS-Medtronic), Medtronic-Minimed, CA, USA, calibrated by the glucometer Calla, Wellion, Austria, and, (2) the Beckman II analyser to measure glucose concentrations in venous plasma. Samples were taken on 4 consecutive days in the fasting state and 4 times after consumption of 50 g glucose. Carelink Personal, MS Excel, Maple and Mat lab were applied to plot the evolution of glucose concentration and analyse the results. The time difference between increase and decrease was calculated for HP, DM 1 and DM 2. Results. In DM1and DM2, glucose tolerance testing (GTT) resulted in slower transport of glucose into subcutaneous tissue than in HP where the lag phase lasted up to 12 min. The maximum increase/decrease rates in DM1 and DM2 vs HP were 0.25 vs < 0.1 mmol/L/min. Conclusion. CGMS is shown to provide reliable plasma glucose concentrations provided the system is calibrated during a steady state. The analysis of glucose change rates improves understanding of metabolic processes better than standard GTT.
Condensation during heat transfer processes can be very beneficially used due to the large amount of energy contained in phase change (vapor to liquid). This contribution focuses on the possible use of polymer hollow fiber heat exchangers (PHFHEs) in air conditioning. PHFHEs consist of hundreds or thousands of polymer hollow fibers with an outer diameter of around 1 mm. The wall thickness is approximately 10% of the outer diameter. PHFHEs are heat exchangers with such benefits as low weight, easy shaping, corrosion resistance, and resistance to many chemical solutions. In comparison with metal heat exchangers (made of copper, aluminum, or stainless steel) the plastic wall of PHFHEs has low thermal conductivity (between 0.1 and 0.4 Wm-1K-1). This seems to be their key disadvantage. However, due to the extremely small thickness of the fiber’s wall this disadvantage is negligible. PHFHEs are compact heat exchangers with a large heat transfer area with respect to their volume. This paper shows the results of condensation tests for PHFHEs that consist of 6 equivalent layers of polypropylene fibers with a length of 190 mm. The total number of fibers is 798. The air humidity was set to 50% with an air temperature of 27°C, which are the typical conditions for such tests in air conditioning technology. Another important parameter was the velocity of the air. Testing velocities were chosen as 3 m s−1 and 1 m s−1. The influence of gravity was studied by putting the PHFHEs in three different positions. The fibers were placed in horizontal and vertical positions, and in a position where fibers form an angle of 45° with the ground. The study showed the ineffectiveness of placing the PHFHE in a horizontal position and suggests that it is better to have a larger distance between the layers of fibers.
This article deals with fatigue tests of polymeric hollow fibre heat transfer surfaces. The hollow fibres have an outer diameter between 0.5-0.8 mm and wall thickness 10 % of the outer diameter. These plastic heat transfer surfaces have some limitations but also many benefits. One of the limitations is the durability of plastic under fatigue loading. The heat transfer surfaces were subjected to pulsating pressure loads under different conditions (level of pressure, ambient temperature, number of cycles). Firstly, only an internal hydraulic pulsating load was applied and the behaviour of the hollow fibres was observed, focusing especially on the presence of leaks, ruptures, etc.Then, other conditions of operations were added. The heat transfer surfaces were immersed in a hot bath and loaded by internal pulsating pressure and high temperature simultaneously. Testing under different temperatures is important because the temperature significantly affects the material properties. The presence of leaks, ruptures and other possible damage was monitored as with previous tests.
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