RADIATION AND CONVECTION IN CONIFERS

The transfer of energy to and from a conifer branch involves solar radiation, thermal radiation from the ground, atmosphere, and surroundings, thermal emission by the branch, and free convection in still air and forced convection in wind. It is necessary to know the actual surface area of the branch, the effective area for absorbing sunlight, the effective area for absorbing long wave thermal radiation and for emission, and the free and forced convection coefficients. These parameters are determined using silver castings of blue spruce and white fir branches suspended in an evacuated radiation chamber and in a wind tunnel. The actual surface area of a branch is determined by means of an electrolytic technique. Numerical examples are given for energy transfer in a natural environment for conifers and comparison is made to a broad deciduous type of leaf. The role of transpiration in the energy transfer process is discussed.     

Foliage influences forced convection heat transfer in conifer branches and buds

Conifer foliage structures affect branch and bud temperature by altering the development and convective resistance of the thermal boundary layer. This paper examines foliage effects on forced convection in branches and buds of Picea glauca (Moench) Voss and Pinus contorta Dougl. Ex. Loud., two species that represent the range of variation in foliage structure among conifers. Forced convection is characterized by a power law relating Nusselt (heat transfer) and Reynolds (boundary layer development) numbers. Data were collected in a laminar flow wind tunnel for free stream velocities of 0.16-6.95 m s(-1). Scaling parameters were compared against literature values for silver cast branch replicas, a bed of real foliage, cylinders, and tube banks. Foliage structures reduced Nusselt numbers (heat transfer) relative to cylinders, which are typically used to approximate leafless branches and buds. Significantly different scaling relationships were observed for all foliage structures considered. Forced convection scaling relationships varied with foliage structure. The scaling relationships reported here account for variation within populations of branches and buds and can be used to characterize forced convection in a forest canopy.     

哺乳动物毛被传热性能及其影响因素

毛被能够加强或减弱动物向周围环境的热量散失,毛被的形态结构和颜色是传热性能的决定因素,其传热过程往往是传导、对流和辐射3个过程的耦合。以往研究发现环境因子中,风可增加机体向环境中的散热速率,且散失量与风速正相关,且动物通过调节在风场中的姿态来适应不同风向。动物体与环境间的温差是影响散热速率的另一因素,不同环境中的动物通过改变毛被结构来适应温差变化。毛被含水率上升会引起导热和蒸发冷却作用加强,动物通过行为或毛被结构变化来调节毛被含水率。毛色决定毛被吸收和反射热辐射的能力。毛被传热性能直接把动物的生理特点与环境因子关联起来,这对揭示动物的适应、进化都具有重要意义。同时提出,毛被结构和传热性能的研究还有助于仿生学意义的挖掘。因此,今后应重点在毛被结构和物理性能、研究技术与方法以及毛被生物学和仿生学意义等方面开展研究。     

Entropy budget of conifer branches

From energy budget data for a branch of ponderosa pine given by Gates, Tibbals and Kreith, entropy fluxes into or out of the branch due to solar radiation, infrared radiation, transpiration and convection are calculated. Net entropy flow into the branch is negative. Assuming that the entropy in the branch is at steady state, the entropy production in the branch of ponderosa pine is calculated and shown to be positive. A positive entropy production indicates that the Second Law of Thermodynamics is certainly valid in the branch. Entropy productions in other conifers, blue spruce and white fir, and in a single pine needle in a horizontal position are also calculated. The entropy production (S_prod) increases linearly with the solar energy absorbed by the plant surface (E_solar); S_prod≈(30.6 E_solar)×10⁻⁴. The ratio (S_prod/E_solar) does not differ between deciduous leaves reported earlier and conifer branches. The theorem of oscillating entropy production proposed earlier holds also for conifer branches and will be of universal nature, applicable to all plant leaves.     

Effect of forced convection on the skin thermal expression of breast cancer

A bioheat-transfer-based numerical model was utilized to study the energy balance in healthy and malignant breasts subjected to forced convection in a wind tunnel. Steady-state temperature distributions on the skin surface of the breasts were obtained by numerically solving the conjugate heat transfer problem. Parametric studies on the influences of the airflow on the skin thermal expression of tumors were performed. It was found that the presence of tumor may not be clearly shown due to the irregularities of the skin temperature distribution induced by the airflow field. Nevertheless, image subtraction techniques could be employed to eliminate the effects of the flow field and thermal noise and significantly improve the thermal signature of the tumor on the skin surface. Inclusion of the possible skin vascular response to cold stress caused by the airflow further enhances the signal, especially for deeply embedded tumors that otherwise may not be detectable.     

Heat transfer in elephants: thermal partitioning based on skin temperature profiles

The elephant with its low surface-to-volume ratio presents an interesting problem concerning heat dissipation. To understand how such large mammals remain in thermal balance, we determined the major avenues of heat loss for an adult African elephant and an immature Indian elephant. Because conventional physiological measurements are difficult for these animals, the present study used a non-invasive technique, infrared thermography, to measure skin temperatures of each elephant. Detailed surface temperature profiles and surface area measurements of each elephant were used in standard equations for convective, conductive and radiant heat transfer. Results demonstrated that heat transfer by free convection and radiation accounted for 86% of the total heat loss for the elephants at T _a= 12·6 °C. Heat transfer across the ears, an important thermal window at high ambient temperatures, represented less than 8% of the total heat loss. Surface area of the animals, and metabolic heat production calculated from total heat loss of the African elephant, scaled predictably with body mass. In contrast, the thermal conductance of the elephants (71·6 W /°C, African; 84·5 W /°C, Indian) was three to five times higher than predicted from an allometric relationship for smaller mammals. The high thermal conductance of elephants is attributed to the absence of fur and appears to counteract reduced heat transfer associated with a low surface-to-volume ratio.     

Monoterpene emission from coniferous trees in response to elevated CO2 concentration and climate warming

It was hypothesized that high CO₂ availability would increase monoterpene emission to the atmosphere. This hypothesis was based on resource allocation theory which predicts increased production of plant secondary compounds when carbon is in excess of that required for growth. Monoterpene emission rates were measured from needles of (a) Ponderosa pine grown at different CO₂ concentrations and soil nitrogen levels, and (b) Douglas fir grown at different CO₂ concentrations. Ponderosa pine grown at 700 μmol mol^–1 CO₂ exhibited increased photosynthetic rates and needle starch to nitrogen (N) ratios when compared to trees grown at 350 μmol mol^–1 CO₂. Nitrogen availability had no consistent effect on photosynthesis. Douglas fir grown at 550 μmol mol^–1 CO₂ exhibited increased photosynthetic rates as compared to growth at 350 μmol mol^–1 CO₂ in old, but not young needles, and there was no influence on the starch/N ratio. In neither species was there a significant effect of elevated growth CO₂ on needle monoterpene concentration or emission rate. The influence of climate warming and leaf area index (LAI) on monoterpene emission were also investigated. Douglas fir grown at elevated CO₂ plus a 4 °C increase in growth temperature exhibited no change in needle monoterpene concentration, despite a predicted 50% increase in emission rate. At elevated CO₂ concentration the LAI increased in Ponderosa pine, but not Douglas fir. The combination of increased LAI and climate warming are predicted to cause an 80% increase in monoterpene emissions from Ponderosa pine forests and a 50% increase in emissions from Douglas fir forests. This study demonstrates that although growth at elevated CO₂ may not affect the rate of monoterpene emission per unit biomass, the effect of elevated CO₂ on LAI, and the effect of climate warming on monoterpene biosynthesis and volatilization, could increase canopy monoterpene emission rate.     

Thermal convection over East Antarctica: Potential microorganism dispersal

The Indian Antarctic station, Maitri is in the Schirmacher oasis of east Antarctica. The oasisis covered with snow/ice; except for the local summer season when it gets deglaciated and exposes the small hilly region. During summer, minute microorganisms are observed near water bodies of this rocky terrain. In the year 1996a monostatic acoustic sounder probed planetary boundary layer dynamics over this region. From the data it is observed that the thermal convection (both free and forced) persist for 5.63% of the time in the whole year. The occurrence of free convection predominates in the local summer season around midday, while sporadic cases of forced convection have been recorded during the autumn and winter seasons. The onset of convection is mainly at 0600 hrs to 1200 hrs, while the cessation period is limited within 1400 hrs to 1900 hrs. The cessation of convection is basically governed by the katabatic wind flow around the Schirmacher region and it indicates that the interior of Antarctica or the polar cap ice becomes cooler much faster than the rocky region of Schirmacher oasis.Examples taken from the literatures on atmospheric structure and their effects on dispersal of microorganisms and their distribution by the wind are discussed. The study of monostatic acoustic sounder for thermal convections/plumes may form input for the study of dispersal, survival, metabolicactivities and dispersion model of microorganisms. The application of convection/plume to aerobiology can also lead to improvements in forecasting, monitoring and understanding the various mechanisms of organism dispersal.     

Computer-based analysis of steady-state and transient heat transfer of small-sized leaves by free and mixed convection

In order to study convective heat transfer of small leaves, the steady‐state and transient heat flux of small leaf‐shaped model structures (area of one side = 1730 mm²) were studied under zero and low (= 100 mm s^?1) wind velocities by using a computer simulation method. The results show that: (1) distinct temperature gradients of several degrees develop over the surface of the model objects during free and mixed convection; and (2) the shape of the objects and onset of low wind velocities has a considerable effect on the resulting temperature pattern and on the time constant τ. Small leaves can thus show a temperature distribution which is far from uniform under zero and low wind conditions. The approach leads, however, to higher leaf temperatures than would be attained by ‘real’ leaves under identical conditions, because heat transfer by transpiration is neglected. The results demonstrate the fundamental importance of a completely controlled environment when measuring heat dissipation by free convection. As slight air breezes alter the temperature of leaves significantly, the existence of purely free convection appears to be questionable in the case of outdoor conditions. Contrary to the prognoses yielded by standard approximations, no quantitative effect of buoyancy on heat transfer under the considered conditions could be detected for small‐sized leaf shapes.     

Photosynthesis and Photoprotection in Overwintering Plants

Abstract: Seasonal differences in the capacity of photosynthetic electron transport, leaf pigment composition, xanthophyll cycle characteristics and chlorophyll fluorescence emission were investigated in two biennial mesophytes ( Malva neglecta and Verbascum thapsus ) that grow in full sunlight, and in leaves/needles of sun and shade populations of several broad-leafed evergreens and conifers (Vinca minor, Euonymus kiautschovicus, Mahonia repens, Pseudotsuga menziesii [Douglas fir], and Pinus ponderosa). Both mesophytic species maintained or upregulated photosynthetic capacity in the winter and exhibited no upregulation of photoprotection. In contrast, photosynthetic capacity was downregulated in sun leaves/needles of V. minor, Douglas fir, and Ponderosa pine, and even in shade needles of Douglas fir. Interestingly, photosynthetic capacity was upregulated during the winter in shade leaves/needles of V. minor, Ponderosa pine and Euonymus kiautschovicus. Nocturnal retention of zeaxanthin and antheraxanthin, and their sustained engagement in a state primed for energy dissipation, were observed largely in the leaves/needles of sun-exposed evergreen species during winter. Factors that may contribute to these differing responses to winter stress, including chloroplast redox state, the relative levels of source and sink activity at the whole plant level, and apoplastic versus symplastic phloem loading, are discussed.     

The Role of the Plumage in Heat Transfer Processes of Birds

The plumage of birds provides a critical thermal buffer betweenthe animal and its environment. Rates of energy expenditureare strongly influenced by the thermal properties of the environmentor the microclimates the animal occupies. Current data suggestthat the addition of solar radiation is equivalent to threeto four-fold changes in wind speed and that solar heat gaincan be extremely sensitive to changes in wind speed. Dry heat transfer through the plumage occurs by three avenues1) conduction and free convection through air 2) conductionalong the solid elements of the plumage and 3) radiation. Overall,about 95% of the total heat flow is evenly divided between thefirst two avenues. Radiative heat transfer accounts for onlyabout 5% of total heat flow. Plumage color, as well as the microstructureand micro-optical properties of plumage elements, when combinedwith environmental properties (e.g., wind speed), determinethe radiative heat loads that birds acquire from solar radiation.Although plumage color or reflectivity determines the fractionof incident solar radiation that is absorbed by the plumageand generates heat, the fraction of this heat that contributesto the thermal load on the animal can vary greatly. In a fibrouscoat such as a plumage, there is some variable penetration intothe coat, with absorption over a range of coat depths. Factorssuch as feather micro-optics and structure are critical determinantsof radiation penetration into avian coats. Significant differencesin solar heat loads can also result from behavioral adjustmentsin plumage thickness.     

Buoyancy effect on forced convection in the leaf boundary layer

Abstract. Mixed convection (forced convection plus free convection) in the leaf boundary layer was examined by air flow visualization and by evaluation of the boundary layer conductance at different leaf-air temperature differences ( T _L- T _A) under low wind velocities. The visualized air flow was found to become more unstable and buoyant at higher T _L- T _A. An ascending longitudinal plume was induced along the upper surface, and the air flow along the lower surface ascended after passing the trailing leaf edge. The air flow modified by buoyancy was considered to result in an increase in boundary layer conductance ( G _A) for mixed convection, which became higher with higher T _L- T _A as compared with the conductance for pure forced convection without buoyancy. This increase in G _A appeared larger at larger Grashof number (Gr) and at smaller Reynolds number (Re). The dependences of buoyancy effect on Gr and Re were related to 'edge-effects'.     

Air flow and convective heat loss from small mammals in laboratory metabolism chambers

1. 1.Thermal conductance was determined from cooling curves of Mus domesticus carcasses at air flow rates ranging from still air to 0.91·min⁻¹ STP. Thermal conductance was constant over this range of air flows. This indicates that free convection predominates over forced convection at these air flow rates.

2. 2.While non single set of conditions will be appropriate for all experiments, free convection conditions are appropriate for determination of minimal thermal conductance.

A thermal model of the human body exposed to an electromagnetic field

The human body was modeled by numerical procedures to determine the thermal response under varied electromagnetic (EM) exposures. The basic approach taken was to modify the heat transfer equations for man in air to account for thermal loading due to the energy absorbed from the EM field. The human body was represented in an EM model by a large number of small cubical cells of tissue, and the energy density was determined for each cell. This information was then analyzed by a thermal response model consisting of a series of two-dimensional transient conduction equations with internal heat generation due to metabolism, internal convective heat transfer due to blood flow, external interaction by convection and radiation, and cooling of the skin by sweating and evaporation. This model simulated the human body by a series of cylindrical segments. The local temperature at 61 discrete locations as well as the thermoregulatory responses of vasodilatation and sweating were computed for a number of EM field intensities and two frequencies, one near whole-body resonance.     

Theoretical and Experimental Studies of Energy Exchange from Jackrabbit Ears and Cylindrically Shaped Appendages

Convection properties of jackrabbit ears were examined in a wind tunnel and in the field in an attempt to study the possible thermal role of the large ears. This work was part of a study on energy exchange of appendages. Cylindrical copper models of various shapes, aluminum castings of domestic and jackrabbit ears, and an amputated jackrabbit ear were studied in a wind tunnel (a) to define the range for convective heat loss for appendages of various shapes, and (b) to study the effect on convection of model shape and orientation to the wind. Shape, i.e. length and closure, proved important. Orientation to the wind produced no consistent or significant variation in the convection coefficient. The convection coefficients from the ear castings fell within the range generated from the cylindrical models. The convection coefficients for the amputated rabbit ear fell partially within the range. Net thermal radiation loss at midday from the jackrabbit ears was found to be small. Convection from the ears, however, could account for the loss of over 100% of the animal's metabolic heat at an air temperature of 30°C. If air temperature exceeds body temperature, the animal must either store heat or resort to the evaporation of water.     

High surface temperatures of trees and pine litter in the winter and their biological importance

Measurements in The Netherlands show that in the winter (Dec.–March) solar radiation measured perpendicular to the solar beam can be quite strong. Consequently, high surface temperatures can occur on suitably exposed, dark surfaces of low thermal conductivity. In December, the surface temperature on the bark of old pine trees was found to be up to 28°C above the ambient air temperature. In February, the excess temperatures of pine bark reached as high as 37°C. The temperatures of steeply south-exposed pine litter were even somewhat higher. South-exposed edges of pinewoods and similar situations are especially favourable due to the wind shelter and extra radiation gain they provide. In February, ants were found to bask in clusters attaining temperatures of as much as 20°C above that of ambient air. Basking vipers attained excess temperatures of 25°C.     

Phytoestrogen from Pinus ponderosa assayed by competitive binding with 17beta-estradiol to mouse uterine cytosol

Ponderosa pine needle extracts contained a phytoestrogen that competed with 17beta-estradiol for specific binding to mouse uterine cytosol. Phytoestrogen was separated from the initial aqueous extract by acetone fractionation and eluted from a polyvinylpyrrolidone column with 90% methanol. Forty mu g/ml was determined as the concentration of most purified phytoestrogen displacing 50% of 17beta-estradiol (15 pg/ml) from the estrogen specific binding sites of the mouse uterine cytosol. Intraperitoneal administration of 200 mug (0.3 g needle equivalents) of the phytoestrogen was equal in activity to 10 ng of 17beta-estradiol in a 24 hour uterine growth assay in immature mice. The phytoestrogen was retained in dialysis tubing with a 14,000 molecular weight cutoff and displayed an ultraviolet absorbance maximum at 208 nm, void of any phenolic chromophores. Phytoestrogen was being evaluated as a contributory factor in Ponderosa pine needle-induced abortion observed in range cattle.     

Convective Cooling at Low Airspeeds and the Shapes of Broad Leaves

Circular, abstractly lobed, and leaf-shaped flat copper plateswere heated in a very low-speed wind tunnel. Surface temperaturedistributions of the plates were matched to those of real leaves.With the centres of the plates 15°C above ambient temperature,their heat dissipation was measured at wind velocities of <1,10, and 30 cm s^–1 from below and laterally with horizontaland vertical plate orientations. Even very slight forced-airmovements markedly increased heat dissipation in this rangeof mixed free and forced convection. Lobed plates were moreeffective dissipators than circles, with the greatest differencesoccurring where flow was normal to the plate. Circular platesdissipated about one-fourth more heat when vertical than whenhorizontal in still air (free convection). By contrast, dissipationwas essentially independent of orientation for extensively lobedmodels. Under some circumstances, maximum dissipation occurredwith lobed plates oblique to a forced air stream. Physical explanationsand biological implications of these results are discussed.     

Air movement and heat loss from sheep. I. Boundary layer insulation of a model sheep, with and without fleece

A model sheep, made from metal cylinders and hemispheres, was heated electrically. Heat loss by forced convection in a wind tunnel was analysed in terms of the dependence of the Nusselt number (Nu) on Reynolds number (Re). For a bare trunk Nu = 0.095 Re0.684, but with fleece covering the trunk to a depth of 3.5 cm, Nu = 0.0112 Re0.875 when the mean radiative temperature of the the coat was taken as the surface temperature. Heat transfer by convection from the whole body, including legs, was described by Nu = 0.029 Re0.80. However, a bulk Nesselt number should not be used to estimate heat loss from a live sheep in a hot environment if the windspeed is below about 4 m s-1 because the relation between mean surface temperature, Nusselt number and convective heat flux is not unique.     

Unsteady Convection Flow and Heat Transfer over a Vertical Stretching Surface

This paper investigates the effect of thermal radiation on unsteady convection flow and heat transfer over a vertical permeable stretching surface in porous medium, where the effects of temperature dependent viscosity and thermal conductivity are also considered. By using a similarity transformation, the governing time-dependent boundary layer equations for momentum and thermal energy are first transformed into coupled, non-linear ordinary differential equations with variable coefficients. Numerical solutions to these equations subject to appropriate boundary conditions are obtained by the numerical shooting technique with fourth-fifth order Runge-Kutta scheme. Numerical results show that as viscosity variation parameter increases both the absolute value of the surface friction coefficient and the absolute value of the surface temperature gradient increase whereas the temperature decreases slightly. With the increase of viscosity variation parameter, the velocity decreases near the sheet surface but increases far away from the surface of the sheet in the boundary layer. The increase in permeability parameter leads to the decrease in both the temperature and the absolute value of the surface friction coefficient, and the increase in both the velocity and the absolute value of the surface temperature gradient.