ice formation around a horizontal tube in a rectangular vessel

Waterglycolsolution410 mm 500 mm 500 mm 470 mm 580 mm 480 mm 500 mm 560 mmFigure 2. Measurement stations along test tubeTw4Tw4Tw3Tw3Tw3Tw6 Tw5Tw2Tw1Tw7 Tw813 10 1 1 10 135 14324275Tw2Tw1143242755Tw6 Tw5Tw2Tw1Tw7 Tw813 10 1 1 10 131 8323975a) x/L= 0.1 b) x/L= 0.47 c) x/L= 0.86Figure 3. Temperature measurement locations within the waterAll the thermocouples used were K type, tefloninsulated thermocouples. An ice-bath was employed asthe reference junction for all the thermocouples used.Outputs of the thermocouples were monitoredcontinuosly with a sampling rate of 30 readings/s, usinga computer-controlled data logging system.Due to the experimental nature of the present work, carewas given to the uncertainty in measurements. The fluidflow rate is measured by means of a calibratedrotometer with accuracy of ± 4%. To performcalibration of the rotometer, water was pumped from asupply reservoir to a constant head reservoir. Fromthere, water flowed to a weighing tank through themeasuring section with the rotometer under test. Theflow rate was adjusted with a control valve situated atthe downstream end of the test section. The rotometerwas calibrated by measuring the volume of water thatpasses the rotometer during a certain measuring time.Table 1. Uncertanity and range for different parametersParameter Range UncertainityTube inner radious (mm) 6 ± 0.1 mmTube outer radious (mm) 8 ± 0.1 mmTube length (m) 0 − 4 ± 0.5 cmFlow rate (l/h) 0 − 1000 ± %4Temperature (ºC) -7 to 30 ± 0.3 ºCIce thickness (mm) 0 − 20 ± 0.1 mmHereto the water was collected in the weighing tank,whereby the volume increment was measured andaverage flow rate was computed by dividing the volumeby the time. Temperatures of water and transfer fluidare measured by means of K type teflon insulatedthermocouples with accuracy ± 0.3 ºC. Thethermocouples were calibrated against a platinumresistance thermometer in a stirred liquid bath whosetemperature was adjusted with the help of atemperature-measuring bench. Thickness of the iceformed around the tube is measured with ± 0.1 mm.Table 1 shows the range and uncertainity of parametersfor the experiments. Thickness of the ice formed on thetube was determined using a visual technique. Icethickness was measured at the same stations wheretemperature measurements were made. Smallobservation windows are opened through the insulationmaterial at the measuring stations to be able to make icethickness measurements. Photographs of the ice weretaken with a digital camera (Nikon Coolpix 8700, 8MP,10x optic zoom) from the front and the top of thevessel at each station to be able to detect circumferentialchanges in ice thickness. Different light sources andpositions were tested during the prelimenary experimetsto determine the best lighting arrangement. A traversingmechanism was designed to be able to detect the iceformation along the tube with a short period of timebetween the stations. The time period between any twomeasuring station was 20 s at maximum. Allphotographs were taken in close-up mode with 5 MPresolution. Figure 4 shows two images of solidificationtaken at two different times at the first measuringstation (x/L=0.1) for an inlet temperature of -5.8 ºC,flow rate 900 l/h as an example to the images takenduring the experimets. At the end of experiments, thephotographs taken were transferred to an image analysisprogram (Image-ProPlus 4.5) for post processing. Usingsome of the techniques offerred by the program, brightimages that clearly indicate the boundaries of the iceformed around the tube were obtained. From the imagesprocessed, thickness of the ice was determined for thetop and bottom of the pipe separately (left and rightsides of the pipe for the photographs taken from the topof the pipe) using the pipe diameter as a referencedimension.78