When ambient temperatures are low, paraffinic crude oils being transported in pipelines may form gels composed of wax crystals. If pipeline flow ceases, these waxy gels may make it difficult to restart the flow without breaking the pipe. To predict the severity of this problem, we consider the rheology of a transparent model waxy crude oil for which pipeline flow visualization results are presented elsewhere. We investigate characteristics of the model oil determined by cone-plate shear flow measurements, such as the viscosity and wax appearance temperature, the gelation temperature, the elastic modulus, and the yielding behavior of the gel. The yielding behavior is a critical determinant of pipeline restart, and the time-dependent yielding behavior observed for this model oil is similar to that reported previously for North Sea crude oils. In particular, at sufficiently low-stress levels, the gel never yields, whereas the gel yields or "fractures" immediately at sufficiently high-stress levels. At intermediate-stress levels, the gel "creeps" with a delay time to fracture that ranges from seconds to hours, depending upon the imposed stress value. Some authors have suggested that waxy gels slowly degrade as they creep and that this gives rise to the very long delay times to fracture that may be observed. However, a creep-response hysteresis test on the model oil studied here shows that the gel elastic modulus does not vary with time during creep, a result which is inconsistent with the degradation mechanism.

in Wiley Online Library (wileyonlinelibrary.com).Paraffinic crude oils in pipelines may form waxy gels during flow shutdowns. These gels can be dislodged by applying pressure if the wall shear stress, proportional to the local pressure gradient, exceeds the gel yield stress. The simplest models assume that the axial pressure profile becomes linear immediately after a jump in upstream pressure, but this fails to account for gel time-dependent rheology or the effect of gel voids on pressure wave propagation. To investigate the former factor, pressure profile and particle imaging velocimetry (PIV) measurements were performed on a model oil gelled under pressure to reduce void formation. After a jump in upstream pressure to a value insufficient to restart flow, the axial pressure profile becomes linear in a twostep process, with an immediate small rise in downstream pressure followed by a timedelayed jump. The local downstream gel deformation measured by PIV exhibits similar two-step time dependence.

This study presents the design and computational fluid dynamics (CFD) analysis of a mini demonstrator for a dual fluid reactor (DFR). The DFR is a novel concept currently under investigation. The DFR is characterized by the implementation of two distinct liquid loops dedicated to fuel and coolant. It integrates the principles of molten salt reactors and liquid metal cooled reactors; thus, it operates in a high temperature and fast neutron spectrum, presenting a distinct approach in the field of advanced nuclear reactor design. The mini demonstrator serves as a scaled-down version of the actual reactor, primarily aimed at gaining insights into the CFD analysis intricacies of the reactor while minimizing computational costs. The CFD modeling of the MD intends to add valuable data for the purpose of modeling validation against experiments to be conducted on the MD. These experiments can be used for DFR licensing and design optimization. The coolant and fuel utilized in the mini demonstrator are of low Prandtl number (Pr = 0.01) liquid lead, operating at two distinct inlet temperatures, namely 873 K and 1473 K. The study showed a rapid increase in turbulence due to intense mixing and abrupt changes in flow areas and directions, despite the relatively low inlet velocities. Hot spots characterized by elevated temperatures were identified, analyzed, and justified based on their spatial distribution and flow conditions. Flow swirling within pipes was identified and a remedy approach was suggested. Inconsistent mass flow rates were observed among the fuel pipes, with higher rates observed in the lateral pipes. Although lower fuel temperatures were observed in the lateral pipes, they consistently exhibited higher heat exchange characteristics. The study concludes by giving physical insights into the heat transfer and flow behavior, and proposing design considerations for the dual fluid reactor to enhance structural safety and durability, based on the preliminary analysis conducted.

Because of limited sound intensity output from commercial drivers at midaudio and ultrasonic frequency ranges used in thermoacoustic coolers, it is important to optimize their performance. To achieve this, studies were conducted on heat transfer at the cold heat exchanger and at the hot heat exchanger. Measurements were taken on stray heat influx to the cooler by mechanisms of convection, conduction, radiation, and streaming, and from the driver. PIV studies show the contributions of streaming, both from the driver and other parts of the devices. A special 4-layer copper mesh heat exchanger was designed and tested to cope with heat pumped acoustically from the cold heat reservoir as well as the streaming from the piezoelectric driver. For typical sound levels of 155–160 dB achieved inside the resonator by commercial bimorph piezoelectric drivers, a cooling power density of 0.5 watts/cm2 was achieved with air at 1 atm. Pressurizing the working gas to 17 atm raises the cooling density. The choice of working gas provides a increase in power density. With the above optimizing approaches, a cooling power density of over 50 watts/cm2 appears feasible for the frequency range of 4 to 24 kHz. [Work supported by a grant from the ONR and the State of Utah.]