AVERAGE HEAT TRANSFER OF TUBES IN DOWNWARD FOAM FLOW

The model of heat exchanger was investigated experimentally. This model consists of three vertical lines of horizontal tubes with five tubes in each. Tubes were arranged in a staggered order. Heat transfer of staggered bundle of tubes to downward static stable foam flow was investigated experimentally. Heat transfer dependence on specific gas and liquid velocity was determined. Dependence of volumetric void fraction of foam on heat transfer was investigated also. Heat transfer rate dependence on tube position in the line of tube bundle was investigated experimentally. It was established that heat transfer rate highly depends on tube position in the line. Influence of tube position on heat transfer from tube bundle in upward foam flow was compared. Heat transfer dependence on tube position in the bundle was investigated experimentally also. Influence of wall of foam generator on heat transfer to sideline of tubes was established. Experimental results of heat transfer of bundle of tubes to downward static stable foam flow were generalized using dependence between Nusselt and Reynolds numbers

Influence of tube bundle geometry on heat transfer to foam flow

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.Usage of two–phase gas–liquid foam flow as a coolant allows achieving relatively large heat transfer intensity with smaller coolant mass flow rate. However number of foam peculiarities complicates an application of the analytical methods for heat transfer investigation. Presently an experimental method of investigation was selected as the most suitable. Due to the fact that tube bundles of different geometry my be used in foam apparatus, the experimental investigation of heat transfer of the in–line tube bundle with different spacing between tubes to vertical foam flow was performed. Spacing among the centres of the tubes across the in–line tube bundle was 0.03 m and spacing along the bundle was 0.03 m. In other case the spacing along the bundle was 0.06 m. Results of investigation showed that an effect of “shadow” is slight and heat transfer is higher for the tubes of the in–line tube bundle with more spacing between the tubes’ centres along the bundle.cs201

Cas3 is a limiting factor for CRISPR-Cas immunity in Escherichia coli cells lacking H-NS

Background: CRISPR-Cas systems provide adaptive immunity to mobile genetic elements in prokaryotes. In many bacteria, including E. coli, a specialized ribonucleoprotein complex called Cascade enacts immunity by “an interference reaction" between CRISPR encoded RNA (crRNA) and invader DNA sequences called “protospacers”. Cascade recognizes invader DNA via short “protospacer adjacent motif” (PAM) sequences and crRNA-DNA complementarity. This triggers degradation of invader DNA by Cas3 protein and in some circumstances stimulates capture of new invader DNA protospacers for incorporation into CRISPR as “spacers” by Cas1 and Cas2 proteins, thus enhancing immunity. Co-expression of Cascade, Cas3 and crRNA is effective at giving E. coli cells resistance to phage lysis, if a transcriptional repressor of Cascade and CRISPR, H-NS, is inactivated (Δhns). We present further genetic analyses of the regulation of CRISPR-Cas mediated phage resistance in Δhns E. coli cells. Results: We observed that E. coli Type I-E CRISPR-Cas mediated resistance to phage λ was strongly temperature dependent, when repeating previously published experimental procedures. Further genetic analyses highlighted the importance of culture conditions for controlling the extent of CRISPR immunity in E. coli. These data identified that expression levels of cas3 is an important limiting factor for successful resistance to phage. Significantly, we describe the new identification that cas3 is also under transcriptional control by H-NS but that this is exerted only in stationary phase cells. Conclusions: Regulation of cas3 is responsive to phase of growth, and to growth temperature in E. coli, impacting on the efficacy of CRISPR-Cas immunity in these experimental systems

CRISPR Interference Directs Strand Specific Spacer Acquisition

Background: CRISPR/Cas is a widespread adaptive immune system in prokaryotes. This system integrates short stretches of DNA derived from invading nucleic acids into genomic CRISPR loci, which function as memory of previously encountered invaders. In Escherichia coli, transcripts of these loci are cleaved into small RNAs and utilized by the Cascade complex to bind invader DNA, which is then likely degraded by Cas3 during CRISPR interference. Results: We describe how a CRISPR-activated E. coli K12 is cured from a high copy number plasmid under non-selective conditions in a CRISPR-mediated way. Cured clones integrated at least one up to five anti-plasmid spacers in genomic CRISPR loci. New spacers are integrated directly downstream of the leader sequence. The spacers are non-randomly selected to target protospacers with an AAG protospacer adjacent motif, which is located directly upstream of the protospacer. A cooccurrence of PAM deviations and CRISPR repeat mutations was observed, indicating that one nucleotide from the PAM is incorporated as the last nucleotide of the repeat during integration of a new spacer. When multiple spacers were integrated in a single clone, all spacer targeted the same strand of the plasmid, implying that CRISPR interference caused by the first integrated spacer directs subsequent spacer acquisition events in a strand specific manner. Conclusions: The E. coli Type I-E CRISPR/Cas system provides resistance against bacteriophage infection, but also enables removal of residing plasmids. We established that there is a positive feedback loop between active spacers in a cluster – i

ANALYSIS OF TUBE BUNDLE HEAT TRANSFER TO VERTICAL FOAM FLOW

Phenomena of foam flow and associated heat transfer are rather complex. Foam is a twophase flow, which structure changes while it passes an obstacle: bubbles divide into smaller bubbles and liquid drains down from flow. Due to these peculiarities, an application of analytical methods for their study is a complex subject. Thus experimental method of investigation was selected in our work. The investigation apparatus consisted of foam generator, vertical channel and staggered bank of horizontal tubes. The cross section of the channel had square profile with side dimension 140 mm. Tubes in the bank were located in three vertical rows with five tubes in each of them. Experiments were performed within Reynolds number diapason for gas from 190 to 450 and foam void volumetric fraction from 0.996 to 0.998. Direction of foam motion in vertical channels also influences heat transfer intensity. Investigations of heat transfer process of upward and downward moving statically stable foam flow from horizontal tube bank were performed. Experimental heat transfer results of tube bundle in vertical cross foam flow were summarized by criterion equations, which enable determination of heat transfer intensity of the entire bundle or of a separate tube of the bundle for different values of void volumetric fractions and regime parameters of statically stable foam flow

Pecularities of heat transfer from in-line tube bundles to upward aqueous foam flow

Four in-line tube bundles with different geometry were investigated for establishing their performance in terms of heat transfer enhancement. Two-phase aqueous foam was used as a coolant. Such coolant was considered, because our previous research showed that large heat transfer intensity may be reached even at small mass flow rate of the foam. Spacing among the centres of the tubes across the first in-line tube bundle was 0. 03 m and spacing along the bundle was 0. 03 m. In the second case spacing among the centres of the tubes across the bundle was 0. 03 m; spacing along the bundle was 0. 06 m. In the third case spacing was accordingly 0. 06 and 0. 03 and in the last case spacing was accordingly 0. 06 m and 0. 06 m. During an experimental investigation it was determined a dependence of heat transfer intensity on flow parameters. The investigation of heat transfer from the bundle to upward vertical foam flow was provided for three different values of foam volumetric void fractions β=0. 996÷0. 998. The velocity of the foam flow was changed from 0. 14 to 0. 30 m/s. The heat transfer coefficient varied from 200 to 2000 W/(m2K) for the above mentioned foam flow parameters