T.H. Van der Meer

Towards convective heat transfer enhancement: surface modification, characterization and measurement techniques

In this work, heat transfer surface modification and heat transfer measurement technique is developed. Heat transfer investigation was aimed to study the effect of carbon nano fibers (extremely high thermal conductive material) on the enhancement level in heat transfer. Synthesis of these carbon nano structures is achieved using thermal catalytic chemical vapor deposition process (TCCVD) on a 50 μm pure nickel (Ni270) wire. The micro wire samples covered with CNF layers were subjected to a uniform flow from a nozzle. Heat transfer measurement was achieved by a controlled heat dissipation through the micro wire to attain a constant temperature during the flow. This measurement technique is adopted from hot wire anemometry calibration method. Synthesis of carbon nano structures, heat transfer surface characterization and measurement technique are evaluated. Preliminary results indicate that an average enhancement in Nusselt Number of 17% is achieved.

Indirect Involvement of Amorphous Carbon Layer on Convective Heat Transfer Enhancement Using Carbon Nanofibers

In this work, an experimental heat transfer investigation was carried out to investigate the combined influence of both amorphous carbon (a-C) layer thickness and carbon nanofibers (CNFs) on the convective heat transfer behavior. Synthesis of these carbon nanostructures was achieved using catalytic chemical vapor deposition process on a 50 μm nickel wire at 650 °C. Due to their extremely high thermal conductivity, CNFs are used to augment/modify heat transfer surface. However, the inevitable layer of a-C that occurs during the synthesis of the CNFs layer exhibits low thermal conductivity which may result in insulating the surface. In contrast, the amorphous layer helps in supporting and mechanically stabilizing the CNFs layer attachment to the polycrystalline nickel (Ni270) substrate material. To better understand the influences of these two layers on heat transfer, the growth mechanism of the CNFs layer and the layer of carbon is investigated and growth model is proposed. The combined impact of both a-C and CNFs layers on heat transfer performance is studied on three different samples which were synthesized by varying the deposition period (16 min, 23 min, and 30 min). The microwire samples covered with CNF layers were subjected to a uniform flow from a nozzle. Heat transfer measurement was achieved by a controlled heat dissipation through the microwire to attain a constant temperature during the flow. This measurement technique is adopted from hot wire anemometry calibration method. Maximum heat transfer enhancement of 18% was achieved. This enhancement is mainly attributed to the surface roughness and surface area increase of the samples with moderate CNFs surface area coverage on the sample.

Water for bioenergy: A global analysis

Agriculture is by far the largest water user. This chapter reviews studies on the water footprints (WFs) of bioenergy (in the form of bioethanol, biodiesel, and heat and electricity produced from biomass) and compares their results with the WFs of fossil energy and other types of renewables (wind power, solar thermal energy, and hydropower). WFs for bioenergy vary, depending on crop type applied, production location, and agricultural practice. The most water-efficient way to generate bioenergy is to use biomass for heat generation, with electricity generation being the second best option. Biofuel production requires roughly twice as much water as bioelectricity. Regarding biofuels, bioethanol has smaller WFs than biodiesel. For example, the WF of rapeseed biodiesel is four times larger than the WF of sugarcane ethanol and seven times larger than the WF of sugar beet ethanol. Global weighted ethanol WFs increase in the order of sugar beet, potato, sugarcane, maize, cassava, barley, rye, paddy rice, wheat, and sorghum and range between 60 and 400 m3/GJ. For sugar beet, maize, and sugarcane, differences between regions are large. The European Union, northern Africa, and the United States have relatively small WFs for ethanol from sugar beet and maize, while eastern Europe has large WFs. Global weighted average biodiesel WFs increase in the following order: palm oil (95 m3/GJ), soybean and rapeseed (400 m3/GJ), and jatropha (570 m3/GJ). Conversely, the WFs of fossil fuels are relatively small. Finally, the WF of hydropower varies widely between 0.5 and 850 m3/GJ. Our results provide new insight into the impacts of bioenergy on the use and pollution of freshwater. This knowledge is a valuable contribution to future research and for policies concerning energy needs, freshwater availability, and the choice whether to allocate water to food or energy production

Influence of Microscale Surface Modification on Impinging Flow Heat Transfer Performance

An experimental approach has been used to investigate the influence of a thin layer of carbon nanotubes (CNTs) on the convective heat transfer performance under impinging flow conditions. A successful synthesis of CNT layers was achieved using a thermal catalytic vapor deposition process (TCVD) on silicon sample substrates. Three different structural arrangements, with fully covered, inline, and staggered patterned layers of CNTs, were used to evaluate their heat transfer potential. Systematic surface characterizations were made using scanning electron microscope (SEM) and confocal microscopy. The external surface area ratio of fully covered, staggered, and inline arrangement was obtained to be 4.57, 2.80, and 2.89, respectively. The surface roughness of the fully covered, staggered, and inline arrangement was measured to be (Sa = 0.365 μm, Sq = 0.48 μm), (Sa = 0.969 μm, Sq = 1.291 μm), and (Sa = 1.668 μm, Sq = 1.957 μm), respectively. On average, heat transfer enhancements of 1.4% and − 2.1% were obtained for staggered and inline arrangement of the CNTs layer. This is attributed to the negligible improvement on the effective thermal resistance due to the small area coverage of the CNT layer. In contrast, the fully covered samples enhanced the heat transfer up to 20%. The deposited CNT layer plays a significant role in reducing the effective thermal resistance of the sample, which contributes to the enhancement of heat transfer.

Direct numerical simulations of flow and heat transfer over a circular cylinder at Re = 2000

Unsteady direct numerical simulations of the flow around a circular cylinder have been performed at Re = 2000. Both two-dimensional and three-dimensional simulations were validated with laminar cold flow simulations and experiments. Heat transfer simulations were carried out and the time-averaged local Nusselt number at the cylinder surface was obtained for various Reynolds numbers. Finally, the heat transfer of 2D and 3D simulations are compared. The average Nusselt numbers were found to be in accordance with empirical correlations. The 3D simulation gives a higher heat transfer due to the captured effects of motions in the spanwise direction compared to the 2D simulation. The irregular fluctuation of surface-averaged Nusselt number can be captured by the 3D simulation, while 2D simulation results show a regular fluctuation corresponding to the shedding from the cylinder, similar to that of a laminar flow.

Convective heat transfer enhancement using Carbon nanofibers (CNFs): influence of amorphous carbon layer on heat transfer performance

In this work, an experimental heat transfer investigation was carried out to investigate the combined influence of both amorphous carbon (a-C) layer thickness and carbon nanofibers (CNFs) on the convective heat transfer behavior. Synthesis of these carbon nano structures was achieved using catalytic chemical vapor deposition process (CCVD) on a 50 μm nickel wire at 650°C. Due to their extremely high thermal conductivity, CNFs are used to augment/modify heat transfer surface. However, the inevitable layer of a-C that occurs during the synthesis of the CNFs layer exhibit low thermal conductivity which may result in insulating the surface. In contrast, the amorphous layer helps in supporting and mechanical stabilizing of the CNFs layer attachment to the polycrystalline nickel (Ni270) substrate material. To better understand the influences of these two layer on heat transfer, the growth mechanism of the CNFs layer and the layer of carbon is investigated and growth model is proposed. The combined impact of both a-C and CNFs layer on heat transfer performance is studied on three different samples which were synthesized by varying the deposition period (16 min, 23min and 30 min). The micro wire samples covered with CNF layers were subjected to a uniform flow from a nozzle. Heat transfer measurement was achieved by a controlled heat dissipation through the micro wire to attain a constant temperature during the flow. This measurement technique is adopted from hot wire anemometry calibration method. Maximum heat transfer enhancement of 18% was achieved. This enhancement is mainly attributed to the surface roughness and surface area increase of the samples with moderate CNFs surface area coverage on the sample.