Heat Transfer Coefficients For Drying Research Articles

Investigation of the heat transfer coefficient during the condensation of small quantities of water vapour from a mixture with a high proportion of non-condensable gas in a horizontal smooth tube

During heat recovery of exhaust gas energy in a shell and tube heat exchanger, the exhaust gas is cooled down and the water vapour content may condense when the corresponding temperature is reached. In this case, a condensation heat exchanger allows recovering the latent heat in addition to the sensible heat and is therefore beneficial for a preferably high heat recovery. The database and algorithms for the design of this kind of apparatus are still nowadays quite week because only little experimental work is realized with condensation of water vapour from a mixture with a very high proportion of non-condensable gas in horizontal tubes. The actual research work improves this database with a considerable amount of experimental data and proposes a correlation for the prediction of the heat transfer coefficient in an exhaust gas heat exchanger in which parts of the gas humidity condense. All experiments were carried out under conditions, which are typical for this kind of heat exchangers, hence the presented results can be used for the design of condensation heat exchangers in industrial heat transfer systems. A gas inlet temperature of 120 °C, a range of the air-steam mixture Reynolds number 11,000 ≦ Re ≦ 31,000, three tube lengths between 1.87 m and 3.04 m, water vapour volume fractions of 7 % to 14.5 % as well as tube diameters of 22 mm and 28 mm were selected to derive the correlation. Furthermore, additional tube lengths (0.574 m, 0.973 m, 1.373 m) were investigated to determine the parameters influencing the condensation rate and comprehensive location profiles over the tube length. A correlation for a correction factor fC is derived, which enables to calculate the increase of the heat transfer coefficient due to condensation from the dry heat transfer coefficient without condensation. This dry heat transfer coefficient is calculated by the measured values in the inlet and outlet for a purely dry air-steam mixture cooling without condensation. The correction factor fC depend on the water vapour volume fraction, the Reynolds number and the geometry ratio between the inner diameter and the tube length.

Energy Metabolism and Evaporative Water Loss in the European Free‐Tailed Bat and Hemprich’s Long‐Eared Bat (Microchiroptera): Species Sympatric in the Negev Desert

We compared the thermoregulatory abilities of two insectivorous bat species, Tadarida teniotis (mean body mass 32 g) and Otonycteris hemprichii (mean body mass 25 g), that are of different phylogenetic origins and zoogeographic distributions but are sympatric in the Negev Desert. At night, both were normothermic. By day, both were torpid when exposed to ambient temperatures (T(a)) below 25 degrees Celsius, with concomitant adjustments in metabolic rate (MR). Otonycteris hemprichii entered torpor at higher T(a) than T. teniotis, and, when torpid, their body temperatures (T(b)) were 1 degrees -2 degrees Celsius and 5 degrees -8 degrees Celsius above T(a), respectively; MR was correspondingly reduced. At night, the lower critical temperature of T. teniotis was 31.5 degrees Celsius, and that of O. hemprichii was 33 degrees Celsius. Mean nocturnal thermoneutral MR of T. teniotis was 37% greater than that of O. hemprichii. At high T(a), evaporative water loss (EWL) increased markedly in both species, but it was significantly higher in T. teniotis above 38 degrees Celsius. In both species, the dry heat transfer coefficient (thermal conductance) followed the expected pattern for small mammals, by day and by night. Total EWL was notably low in normothermic and torpid animals of both species, much lower than values reported for other bats, indicating efficient water conservation mechanisms in the study species. Comparing thermoregulatory abilities suggests that O. hemprichii is better adapted to hot, arid environments than T. teniotis, which may explain its wider desert distribution. By both standard and phylogenetically informed ANCOVA, we found no differences in basal metabolic rate (BMR) between desert and nondesert species of insectivorous bats, substantiating previous studies suggesting that low BMR is a characteristic common to insectivorous bats in general.

PHENOTYPIC VARIATION OF LARKS ALONG AN ARIDITY GRADIENT: ARE DESERT BIRDS MORE FLEXIBLE?

We investigated interindividual variation and intra-individual phenotypic flexibility in basal metabolic rate (BMR), total evaporative water loss (TEWL), body temperature (Tb), the minimum dry heat transfer coefficient (h), and organ and muscle size of five species of larks geographically distributed along an aridity gradient. We exposed all species to constant environments of 15°C or 35°C, and examined to what extent interspecific differences in physiology can be attributed to acclimation. We tested the hypothesis that birds from deserts display larger intra-individual phenotypic flexibility and smaller interindividual variation than species from mesic areas. Larks from arid areas had lower BMR, TEWL, and h, but did not have internal organ sizes different from birds from mesic habitats. BMR of 15°C-acclimated birds was 18.0%, 29.1%, 12.2%, 25.3%, and 4.7% higher than of 35°C-acclimated Hoopoe Larks, Dunn's Larks, Spike-heeled Larks, Skylarks, and Woodlarks, respectively. TEWL of 15°C-acclimated Hoopoe Larks exceeded values for 35°C-acclimated individuals by 23% but did not differ between 15°C- and 35°C-acclimated individuals in the other species. The dry heat transfer coefficient was increased in 15°C-acclimated individuals of Skylarks and Dunn's Larks, but not in the other species. Body temperature was on average 0.4°C ± 0.15°C (mean ± 1 sem) lower in 15°C-acclimated individuals of all species. Increased food intake in 15°C-acclimated birds stimulated enlargement of intestine (26.9–38.6%), kidneys (9.8–24.4%), liver (16.5–27.2%), and stomach (22.0–31.6%). The pectoral muscle increased in 15°C-acclimated Spike-heeled Larks and Skylarks, remained unchanged in Hoopoe Larks, and decreased in 15°C-acclimated Woodlarks and Dunn's Larks. We conclude that the degree of intra-individual flexibility varied between physiological traits and among species, but that acclimation does not account for interspecific differences in BMR, TEWL, and h in larks. We found no general support for the hypothesis that species from desert environments display larger intra-individual phenotypic flexibility than those from mesic areas. The coefficient of variation of larks acclimated to their natural environment was smaller in species from arid areas than in species from mesic areas for mass-corrected BMR and surface-specific h, but not for mass-corrected TEWL. The high repeatabilities of BMR, TEWL, and h in several species indicated a within-individual consistency on which natural selection could operate.

Performance of Dehumidifying Heat Exchangers With and Without Wetting Coatings

Limited previous work has shown that use of special hydrophilic coatings will provide lower air pressure drop in finned tube heat exchangers operated under dehumidifying conditions. However, no detailed work has been reported on the effect of different coating types, or different fin surface geometries on the wet pressure drop. In this study, wind tunnel tests were performed on three different fin geometries (wavy, lanced, and louver) under wet and dry conditions. All dehumidification tests were done for fully wet surface conditions. For each geometry, the tests were performed on uncoated and coated heat exchangers. For all three fin geometries, the wet-to-dry pressure drop ratio was 1.2 at 2.5 m/s frontal air velocity. The coatings have no influence on the wet or dry heat transfer coefficient. However, the wet surface heat transfer coefficient was 10 to 30 percent less than the dry heat transfer coefficient, depending on the particular fin geometry. The effect of the fin press oil on wet pressure drop was also studied. If the oil contains a surfactant, good temporary wetting can be obtained on an uncoated surface; however, this effect is quickly degraded as the oil is washed from the surface during wet operation. This work also provides a critical assessment of data reduction methods for wet surface operation, including calculation of the fin efficiency.

The Role of Hyperthermia in the Water Economy of Desert Birds

A number of authors have suggested that hyperthermia, the elevation of body temperature (Tb) 2 degrees-4 degrees C above normal, contributes to a reduction in total evaporative water loss (TEWL) in birds. Information about the role of hyperthermia in the water economy of birds is scattered throughout the literature. We purposed to collate the available information on this subject, to reevaluate the benefits and costs of this process, and to assess its net effect on the water economy of birds, especially species living in deserts. In this review, we first consider the current model of heat balance in birds at high ambient temperatures (Ta), and show that, in most studies performed at a high Ta, birds were increasing their Tb, a violation of the assumption of steady state conditions. Next, we incorporate the rate of heat gain into calculations of the dry heat transfer coefficient (h), below and above temperature equality (Ta=Tb). We develop a method to calculate h at Ta=Tb, using l'Hôpital's rule. The combined result of our approach suggests that birds increase their dry heat transfer even when Ta is above Ta=Tb, contrary to our prediction. Finally, we explore aspects of hyperthermia that reduce water loss, such as an improved thermal gradient and heat storage, and aspects that may augment water loss, the latter a result of increased respiratory water loss when Tb is elevated. Our analysis of the combination of these three factors suggests that, during acute exposure to high Ta (1 h), birds over a size range of 10-1,000 g save about 50% of their TEWL by becoming hyperthermic. For chronic episodes of high Ta (5 h), small birds save water by hyperthermia but large birds do not.

Acclimatization of Thermoregulation in the Desert Cottontail, Sylvilagus audubonii

Seasonal physiological responses to ambient temperature were determined in Sylvilagus audubonii at Tucson, Arizona. From winter to summer the thermoneutral zone of desert cottontails (700 grams) shifted upwards, basal metabolism decreased by 18 per cent, and standard metabolism decreased above thermoneutrality and increased below. Water loss during summer was higher than in winter below 30° C, began to increase at a higher ambient temperature, and rose less steeply. The per cent of metabolic heat production dissipated by evaporative heat loss did not change seasonally, and equalled 100 at an ambient temperature of 41.4° C. Body temperature was 38.3° C at ambient temperatures below 30° C, and did not differ seasonally. Hyperthermia began within thermoneutrality at a slightly higher ambient temperature during summer resulting in lower hyperthermic body temperatures. Body temperature equalled ambient at 41.9° C during summer and a predicted 42.9° C during winter. The dry heat transfer coefficient was higher during summer below 30° C but there was no apparent seasonal difference at higher ambient temperatures. Sylvilagus audubonii survives in the desert by taking advantage of every possibility to minimize the heat load and water expenditure. A relatively high evaporative cooling capacity and high lethal body temperature of 44.8° C provides a safety factor for desert cottontails if avoidance is not possible.