Numerical Investigation of Carbon Dioxide and POE Oil Mixture Behavior in an Accumulator in Trans-critical Heat Pump Mode

Carbon dioxide is one of the most promising next-generation refrigerants for air-conditioning applications in electric vehicles because it has the advantages of good heating performance in cold climates and environmental-friendly properties. Nonetheless, there are limited data about reversible CO2 heat pump systems for component-level performance and design. This paper focuses on the impact of different sizes of oil bleed hole on the liquid level and local pressure drop in the accumulator through CFD simulation. Also, we applied the experimental data obtained with a full CO2 HP system for the boundary conditions and the measured properties of liquid POE-CO2 for the materials in the simulation. The cases with different sizes of the oil bleed hole are investigated using the commercial computational fluid dynamics software Fluent. The results of this paper can be used to design the accumulator and improve 1-D models for reversible systems so that it can operate well in both air-conditioning and heat pump modes

The Performance of an Automotive Carbon Dioxide Heat Pump System in Frosting and Defrosting

This paper presents the performance in frosting and defrosting of an automotive carbon dioxide (CO2) heat pump system, which is composed of a microchannel indoor and outdoor coil, a reciprocating compressor, an electronic expansion valve, and an integrated accumulator and internal heat exchanger. The effects of the criterion for the start of defrosting on the heat pump system performance were investigated experimentally. The defrost-start criterion was determined based on the reduction in the performance of the heat pump system instead of only considering the performance of the outdoor coil. Results show the heat pump system can operate for 64 minutes at 0℃ and 90% RH with a defrost-start criterion of 10 times the initial air-side pressure drop. The accumulator and internal heat exchanger improve system efficiency during frosting conditions. The mass of frost accumulation increases faster in the first 40 minutes and slower in the next 24 minutes for 0℃ and 90% RH. The retained water on the outdoor coil gets saturated after three short frosting/ defrosting cycles or a 64-minute long run. Also, the frosting and defrosting processes are monitored by a web camera to investigate the frost distribution on the two-pass outdoor microchannel heat exchanger. The results are presented for evaluating the frosting distribution and the effects on the overall performance of the heat pump system

Existence of time periodic solutions for a class of non-resonant discrete wave equations

The final publication is available at Springer via http://dx.doi.org/10.1186/s13662-015-0457-zIn this paper, a class of discrete wave equations with Dirichlet boundary conditions are obtained by using the center-difference method. For any positive integers m and T, when the existence of time mT-periodic solutions is considered, a strongly indefinite discrete system needs to be established. By using a variant generalized weak linking theorem, a non-resonant superlinear (or superquadratic) result is obtained and the Ambrosetti-Rabinowitz condition is improved. Such a method cannot be used for the corresponding continuous wave equations or the continuous Hamiltonian systems; however, it is valid for some general discrete Hamiltonian systems

The Role of Internal Heat Exchanger in an R744 Vapor Compression System in the Heat Pump Mode Under Various Conditions

In this paper, the role of the internal heat exchanger (IHX) in a reversible R744 vapor compression system in the heating mode was investigated experimentally and presented. At the first, the optimal refrigerant charge amount for both basic and the IHX systems was determined. Also, the control equations of HPF-maximizing high side pressure were developed and used for both systems. Then, well-tuned systems working at the optimal discharge pressure were compared in two scenarios under various conditions. The results of the experiments reveal the potential benefits of introducing IHX for the R744 system in the heating mode. The efficiency was improved by up to 10.5% by introducing the internal heat exchanger. Also, the benefits of IHX in the frosting conditions were discussed

Sheathing Overlapping and Attachment Methods for Cold-Formed Steel Shear Walls with Corrugated Steel Sheathing

Cold-formed steel (CFS) shear walls sheathed with corrugated steel sheathing are a feasible solution to non-combustible high structural performance CFS shear walls in mid-rise buildings. Corrugated steel sheathings have high in-plane strength and stiffness due to the cross sectional shape of the sheet. This paper presents an experimental study on two specific issues: (1) the sheathing overlapping configurations and their impact to the shear wall performance, (2) the attachment method for sheathing to framing. For the overlapping issue, one overlap and two overlaps in the corrugated sheets were experimentally investigated, it was found that the overlap differences did not cause significant different behaviors and strength of CFS shear walls. For the sheathing-to-framing attachment method, self-drilling screws and dual spot welding were studied. A portable spot welder with dual heads was used in this research. Connection tests and full scale shear wall tests were conducted to study the two different connection methods. It was found that the dual spot welding yielded a weaker connection than the conventional self-drilling screw connections. This paper presents the details of the test programs, research findings and recommendations for CFS shear wall applications

Strength of Cold-Formed Steel Clip Angle in Combined Bending and Shear Loading

Thin-walled cold-formed steel (CFS) clip angles have been commonly used for connecting CFS framing members or attaching CFS members to the major building structure. The implementation of clip angles involves consideration of ultimate strength for combined bending moments and shear forces. Therefore, a test program of CFS clip angle was recently conducted to investigate the behavior and strength of cold-formed steel clip angle subjected to combined bending moments and shear forces at different boundary conditions. The research included connection tests on clip angle. The testing method was adopted from the AISI S914 Test Standard for Joist Connectors Attached to Clod-Formed Structural Framing. This paper presents the details of the test program, test results as well as recommendations for CFS clip angle configurations

Positive solutions for second-order differential equations with singularities and separated integral boundary conditions

We study the existence of positive solutions for second-order differential equations with separated integral boundary conditions. The nonlinear part of the equation involves the derivative and may be singular for the second and third space variables. The result ensures existence of a positive solution when the parameters are in certain ranges. The proof depends on general properties of the associated Green’s function and the Krasnosel’skii–Guo fixed point theorem applied to a perturbed Hammerstein integral operator. Both numerical and analytical examples are constructed to illustrate applications of the theorem to a group of equations. The result generalizes previous work

A complete classification of shuffle groups

For positive integers $k$ and $n$, the shuffle group $G_{k,kn}$ is generated by the $k!$ permutations of a deck of $kn$ cards performed by cutting the deck into $k$ piles with $n$ cards in each pile, and then perfectly interleaving these cards following certain order of the $k$ piles. For $k=2$, the shuffle group $G_{2,2n}$ was determined by Diaconis, Graham and Kantor in 1983. The Shuffle Group Conjecture states that, for general $k$, the shuffle group $G_{k,kn}$ contains $\mathrm{A}_{kn}$ whenever $k\notin\{2,4\}$ and $n$ is not a power of $k$. In particular, the conjecture in the case $k=3$ was posed by Medvedoff and Morrison in 1987. The only values of $k$ for which the Shuffle Group Conjecture has been confirmed so far are powers of $2$, due to recent work of Amarra, Morgan and Praeger based on Classification of Finite Simple Groups. In this paper, we confirm the Shuffle Group Conjecture for all cases using results on $2$-transitive groups and elements of large fixed point ratio in primitive groups