Li-Wu Fan

Experimental Measurements of Thermal Conductivity of Wood Species in China: Effects of Density, Temperature, and Moisture Content

Experimental measurements of thermal conductivity of wood were performed using the heat flow meter and transient plane source technique. The specimens were prepared from five species of both softwoods and hardwoods widely available and used in China, with a wide range of density and moisture content. The transverse thermal conductivity of ovendry specimens is presented as a function of density and temperature up to 90°C and is compared with that along the grain direction for two select species. The influence of moisture content up to 23 percent, which is below the typical fiber saturation point of wood, on the transverse thermal conductivity is presented as well. It is shown that the transverse thermal conductivity of wood increases with density, temperature, and moisture content. Linear correlating equations are proposed in terms of these factors.

A Parametric Study of Prandtl Number Effects on Laminar Natural Convection Heat Transfer From a Horizontal Circular Cylinder to Its Coaxial Triangular Enclosure

A parametric study of Prandtl number effects on laminar natural convection heat transfer in a horizontal equilateral triangular cylinder with a coaxial circular cylinder is conducted. The Prandtl number is varied over a wide range from 10−2 to 105, which corresponds to a variety of working fluids. The governing equations with the Boussinesq approximation for buoyancy are iteratively solved using the finite volume approach. It is shown that the flow patterns and temperature distributions are unique for low-Prandtl-number fluids (Pr ≤ 0.1), and are nearly independent of Prandtl number when Pr ≥ 0.7. In addition, the inclination angle of the triangular enclosure is found to noticeably affect the variations of the local Nusselt number, and to have insignificant influence on the average Nusselt numbers for low Rayleigh numbers when Pr ≥ 0.7.

Thermal Conductivity Enhancement of Ethylene Glycol-Based Suspensions in the Presence of Silver Nanoparticles of Various Shapes

In this technical brief, the effect of adding silver (Ag) nanoparticles of various shapes on the thermal conductivity enhancement of ethylene glycol (EG)-based suspensions was investigated experimentally. These included Ag nanospheres (Ag NSs), Ag nanowires (Ag NWs), and Ag nanoflakes (Ag NFs). Measurements of the thermal conductivity of the suspensions were performed from 10 to 30 °C at an increment of 5 °C. It was shown that the thermal conductivity of the EG-based suspensions increases with raising the temperature. The Ag NWs of a high aspect ratio (∼500) caused greatest relative enhancement up to 15.6% at the highest loading of nearly 0.1 vol. %, whereas the other two shapes of nanoparticles, Ag NSs and Ag NFs with much smaller aspect ratios, only led to enhancements up to 5%. The formation of a network of Ag NWs that facilitates heat conduction was likely responsible for their better performance. The relative enhancement was also predicted by the Hamilton-Crosser model that takes the particle shape effect into consideration. It was shown that the predictions far underestimate the thermal conductivity enhancements but are qualitatively consistent with their shape dependence. As a penalty, however, the presence of Ag NWs was shown to give rise to significant increase in the viscosity of the EG-based suspensions.

Expedited Freezing of Nanoparticle-Enhanced Phase Change Materials (NEPCM) Exhibited Through a Simple 1-D Stefan Problem Formulation

An analytic/integral approach is utilized to solve a model 1-dimensional Stefan problem for a nanofluid that undergoes freezing. Initially, the isothermal nanofluid is contained in a finite slab. During the freezing process, the traveling interface separates the liquid and solid phases that possess their respective thermophysical properties. The most favorable feature of this model is that the thermal property jumps between the liquid and solid phases are accounted for. The problem is made dimensionless and is shown to depend on the thermal conductivity ratio, thermal diffusivity ratio, Stefan (Ste) and subcooling numbers. The energy equation within the solid layer is solved exactly and that of the liquid layer is solved using the integral method. The instantaneous interface position and the moving front velocity are obtained and the total freezing time is then determined. Combinations of two base PCM (water and cyclohexane) and four nanoparticles (alumina, copper, copper oxide and titanium oxide) are chosen for demonstration purposes. The thermal properties of the resulting nanofluids as a function of the volume fraction were determined using models proposed in the literature. The results show that the dimensionless freezing time is independent of the nanofluid constituents and only depends on the volume fraction. Keeping everything else the same, the freezing time is shown to decrease as the volume fraction of the nanoparticle is raised.© 2009 ASME

Enhanced Critical Heat Flux During Quenching of Extremely Dilute Aqueous Colloidal Suspensions With Graphene Oxide Nanosheets

In this Technical Brief, we report on preliminary results of an experimental investigation of quenching of aqueous colloidal suspensions with graphene oxide nanosheets (GONs). Extremely dilute suspensions with only 0.0001% and 0.0002% (in mass fraction) of GONs were studied and their critical heat fluxes (CHF) during quenching were determined to increase markedly by 13.2% and 25.0%, respectively, as compared to that of pure water. Such efficient CHF enhancement was interpreted to be caused primarily by the improved wettability of the quenched surfaces, due to deposition of the fish-scale-shaped GONs resulting in self-assembly quasi-ordered microscale morphologies.

Experimental Verification of Expedited Freezing of Nanoparticle-Enhanced Phase Change Materials (NEPCM)

Highly-conductive nano-sized particles are dispersed into phase change materials (PCM) to improve their effective thermal conductivity, thus leading to suspensions that are referred to as nanoparticle-enhanced PCM (NEPCM). In order to assess the extent of expedited phase change due to the enhanced thermal conductivity, the one-dimensional unidirectional freezing process of NEPCM in a finite slab was investigated experimentally. Thermocouple readings were recorded at several equally-spaced locations along the freezing direction in order to monitor the progress of the freezing front. As an example, cyclohexane (C6 H12 ) and copper oxide (CuO) nanoparticles were chosen to develop the NEPCM with three different volume fractions (0.5, 1.0, and 2.0 vol%). It was shown that the freezing rate for the 0.5 vol% NEPCM is considerably raised as compared to pure cyclohexane. However, further increase of the fraction of nanoparticles to 1.0 and 2.0 vol% did not linearly expedite freezing. Significant sedimentation of nanoparticles was observed for the 2.0 vol% NEPCM. Additionally, in this case the undesirable supercooling phenomenon was enhanced, which suppresses the growth rate of the solidified NEPCM.Copyright

Subcooled Pool Film Boiling Heat Transfer From Spheres With Superhydrophobic Surfaces: An Experimental Study

Pool film boiling was studied by visualized quenching experiments on stainless steel spheres in water at the atmospheric pressure. The surfaces of the spheres were coated to be superhydrophobic (SHB), having a static contact angle greater than 160 deg. Subcooled conditions were concerned parametrically with the subcooling degree being varied from 0 °C (saturated) to 70 °C. It was shown that film boiling is the overwhelming mode of heat transfer during the entire course of quenching as a result of the retention of stable vapor film surrounding the SHB spheres, even at very low wall superheat that normally corresponds to nucleate boiling. Pool boiling heat transfer is enhanced with increasing the subcooling degree, in agreement with the thinning trend of the vapor film thickness. The heat flux enhancement was found to be up to fivefold for the subcooling degree of 70 °C in comparison to the saturated case, at the wall superheat of 200 °C. A modified correlation in the ratio form was proposed to predict pool film boiling heat transfer from spheres as a function of the subcooling degree.

Pool boiling heat transfer and quench front velocity during quenching of a rodlet in subcooled water: Effects of the degree of subcooling

ABSTRACT Pool boiling heat transfer and quench front propagation were investigated during quenching of cylindrical stainless steel rodlets in subcooled water. The degree of subcooling was varied from 0°C (saturated) to 40°C at an increment of 10°C at atmospheric pressure. The results showed that the increase of degree of subcooling accelerates quenching, with the total quenching time being shortened from 90 second (saturated) to 12 second (subcooled by 40°C). As revealed by the boiling curves that were obtained via solving an inverse heat conduction problem in cylindrical coordinates, boiling heat transfer is enhanced significantly for all boiling modes with raising the degree of subcooling. At the highest degree of subcooling of 40°C, the critical heat flux is improved by nearly 300% as compared to that in saturated water. In addition, the rewetting temperature (i.e., Leidenfrost point) was found to increase as a nearly linear function of the degree of subcooling. The quench front was observed to propagate upward from the bottom of the rodlet, which is accelerated noticeably with increasing the degree of subcooling. The average quench front velocity was shown to agree well with the predicted value of an existing theoretical model that was modified to take the influence of subcooling into consideration.

Heat Transfer During Constrained Melting of Graphite-Based Nanofluids in a Spherical Capsule

Transient heat transfer during constrained melting of graphite-based solid-liquid phase change nanofluids in a spherical capsule was investigated experimentally. Nanofluids filled with self-prepared graphite nanosheets (GNSs) were prepared at various loadings up to 1% by weight, using a straight-chain saturated fatty alcohol, i.e., 1-dodecanol (C12H26O), with a nominal melting point of 22 °C as the base fluid. In-house measured thermal properties were adopted for data reduction, including thermal conductivity, dynamic viscosity, latent heat of fusion, specific heat capacity and density. A proper experimental approach depended on volume expansion was figured out to monitor the melting process of nano-enhanced phase change fluid in a spherical capsule indirectly and qualitatively characterize the process. A variety of boundary temperatures were also adopted to vary the intensity of natural convection. It was shown that under low boundary temperatures, a monotonous melting acceleration came into being while increasing the loading due to the monotonously increased thermal conductivity of the nanofluids. While increasing the boundary temperature leads to more intensive natural convection that in turn slowed down melting under the influence of nanoparticles because the contribution by natural convection is significantly suppressed by the dramatically grown dynamic viscosity, e.g., more than 60-fold increase at the loading of 1 wt.%. The melting rate is determined by the competition between the enhanced heat conduction and deteriorated natural convection.Copyright

Constrained melting heat transfer of a phase change material in a finned spherical capsule

Spherical capsules are widely used as housing for phase change materials (PCMs) in heat exchangers for thermal energy storage applications. In view of the low thermal conductivity of common PCM candidates, extended surfaces, such as fins, are routinely added to PCM capsules to improve the thermal performance. In this paper, to quantitatively evaluate the influence of fins on the thermal performance of PCM-filled spherical capsules, constrained melting heat transfer of a PCM in a circumferentially finned spherical capsule with various fin heights was investigated both numerically and experimentally under constant-temperature boundary conditions. The enthalpy-based model was used in the numerical simulations to deal with phase change, while the control volume method was used to solve the governing equations for the melting problem with natural convection in the liquid phase. A spherical melting facility that allows for direct observation of the solid-liquid phase interface during melting was designed and constructed. The spherical capsule and fins were made of glass and aluminum, respectively. Octadecane with a nominal melting point at 28.2°C was used as the PCM. Qualitative agreement was obtained between the experimentally observed and numerically predicted results of solid-liquid interface evolution. The sources of the differences were identified to be the departure of the physical model from the real system, as well as simplifications of the numerical model. The evolution of the natural convective flow and temperature fields during melting are presented in the form of a series of snapshots of streamline and isotherm contours. The heat transfer mechanisms during melting were interpreted by these contour snapshots. The presence of circumferential fins not only enhances heat conduction, but also augments natural convection heat transfer in localized areas in the vicinity of the fins. The localized interactions between these two effects are significantly affected by the fin height. Under the specific conditions considered, fins with height-to-radius ratios of 0.25, 0.50, and 0.75 decrease the total melting time by 10.6%, 20.2%, and 28.7%, respectively, leading to a significant increase of the thermal energy storage rates in the spherical capsule. Although the presence of fins benefits heat transfer enhancement, a more comprehensive understanding of the effects of other important factors, such as the inclination angle of the spherical capsule, is required.