Search

The thymus medulla is a key site for immunoregulation and tolerance, and its functional specialisation is achieved through the complexity of medullary thymic epithelial cells (mTEC). While the importance of the medulla for thymus function is clear, the production and maintenance of mTEC diversity remains poorly understood. Here, using ontogenetic and inducible fate-mapping approaches, we identify mTEC-restricted progenitors as a cytokeratin19+ (K19+) TEC subset that emerges in the embryonic thymus. Importantly, labelling of a single cohort of K19+ TEC during embryogenesis sustains the production of multiple mTEC subsets into adulthood, including CCL21+ mTEClo, Aire+ mTEChi and thymic tuft cells. We show K19+ progenitors arise prior to the acquisition of multiple mTEC-defining features including RANK and CCL21 and are generated independently of the key mTEC regulator, Relb. In conclusion, we identify and define a multipotent mTEC progenitor that emerges during embryogenesis to support mTEC diversity into adult life., Nature Communications, 14 (1), ISSN:2041-1723

In the thymus, cortical thymic epithelial cells (cTECs) and medullary thymic epithelial cells support αβT cell development from lymphoid progenitors. For cTECs, expression of a specialized gene signature that includes Cxcl12, Dll4, and Psmb11 enables the cortex to support T lineage commitment and the generation and selection of CD4⁺CD8⁺ thymocytes. Although the importance of cTECs in T cell development is well defined, mechanisms that shape the cTEC compartment and regulate its functional specialization are unclear. Using a Cxcl12ᴰˢᴿᵉᵈ reporter mouse model, we show that changes in Cxcl12 expression reveal a developmentally regulated program of cTEC heterogeneity. Although cTECs are uniformly Cxcl12ᴰˢᴿᵉᵈ⁺ during neonatal stages, progression through postnatal life triggers the appearance of Cxcl12ᴰˢᴿᵉᵈ⁻ cTECs that continue to reside in the cortex alongside their Cxcl12ᴰˢᴿᵉᵈ⁺ counterparts. This appearance of Cxcl12ᴰˢᴿᵉᵈ⁻ cTECs is controlled by maturation of CD4⁻CD8⁻, but not CD4⁺CD8⁺, thymocytes, demonstrating that stage-specific thymocyte cross-talk controls cTEC heterogeneity. Importantly, although fate-mapping experiments show both Cxcl12ᴰˢᴿᵉᵈ⁺ and Cxcl12ᴰˢᴿᵉᵈ⁻ cTECs share a common Foxn1⁺ cell origin, RNA sequencing analysis shows Cxcl12ᴰˢᴿᵉᵈ⁻ cTECs no longer express Foxn1, which results in loss of the FOXN1-dependent cTEC gene signature and may explain the reduced capacity of Cxcl12ᴰˢᴿᵉᵈ⁻ cTECs for thymocyte interactions. In summary, our study shows that shaping of the cTEC compartment during the life course occurs via stage-specific thymocyte cross-talk, which drives loss of Foxn1 expression and its key target genes, which may then determine the functional competence of the thymic cortex., The Journal of Immunology, 210 (1), ISSN:0022-1767, ISSN:1048-3233, ISSN:1047-7381, ISSN:1550-6606

Packed-bed thermal stores are integral components in numerous bulk electricity storage systems and may also be integrated into renewable generation and process heat systems. In such applications, the store may undergo charging and discharging periods of irregular durations. Previous work has typically concentrated on the initial charging cycles, or on steady-state cyclic operation. Understanding the impact of unpredictable charging periods on the storage behavior is necessary to improve design and operation. In this article, the influence of the cycle duration (or ‘partial-charge’ cycles) on the performance of such thermal stores is investigated. The response to perturbations is explained and provides a framework for understanding the response to realistic load cycles. The packed beds considered here have a rock filler material and air as the heat transfer fluid. The thermodynamic model is based on a modified form of the Schumann equations. Major sources of exergy loss are described, and the various irreversibility generating mechanisms are quantified. It is known that repeated charge-discharge cycles lead to steady-state behavior, which exhibits a trade-off between round-trip efficiency and stored exergy, and the underlying reasons for this are described. The steady state is then perturbed by cycles with a different duration. Short duration perturbations lead to a transient decrease in exergy losses, while longer perturbations increase it. The magnitude of the change in losses is related to the perturbation size and initial cycle period, but changes of 1–10 % are typical. The perturbations also affect the time to return to a steady-state, which may take up to 50 cycles. Segmenting the packed bed into layers reduces the effect of the perturbations, particularly short durations. Operational guidelines are developed, and it is found that packed beds are more resilient to changes in available energy if the store is not suddenly over-charged (i.e. longer perturbations), and if the steady-state cycle duration is relatively long. Furthermore, using the gas exit temperature to control cycle duration reduces the impact of perturbations on the performance, and reduces the time to return to steady-state operation.

Understanding the exergy losses stemming from heat transfer in compressors and expanders is important for many energy storage applications such as compressed air and pumped thermal storage. In order to obtain a better understanding of these losses, CFD simulations were performed for simple gas springs, for a gas spring with an internal grid to mimic valve flow, and for a reciprocating compressor with functioning inlet and outlet valves. The wall heat exchanges for these three cases were examined and compared. The model adopted has previously been validated for a simple gas spring using experimental data from literature. For the gas spring with an internal grid it was found that increased mixing leads to higher heat-transfer-induced hysteresis losses and (at high piston speeds) to a significant pressure loss. These two types of loss can be distinguished by undertaking adiabatic-wall calculations. For a compressor (i.e., with valve flows) heat transfer over the cycle depends very much on valve timing. For example, at 1500 rpm, when the delivery valve is opened at 7 bar the heat transfer coefficient for the initial stages of compression is similar to that for a simple gas spring, whereas for the same speed at 6 bar it is more than doubled.

Renewable energy sources, such as solar-thermal or geothermal heat, and low-/medium-grade industrial waste-heat can be converted into useful power and/or heating with a variety of technologies, including organic Rankine cycle (ORC ) and vapour-compression heat-pump systems. The thermodynamic performance and cost of these technologies depends crucially on the efficiency of key components, including the compressor or expander used. Reciprocating-piston machines can be advantageous over turbomachines and other positive- displacement machines at intermediate scales (~1 0s-100s of kW) thanks to their ability to operate with relatively high isentropic efficiencies at large expansion ratios. However, modelling the thermodynamic losses in reciprocating-piston expanders, with a view towards designing high-performance machines, is a complex undertaking. The aim of this paper is to develop a spatially-lumped, yet dynamic model of a piston expander suitable for early-stage engineering design, that can provide simplification without sacrificing accuracy. The unsteady heat transfer between the gas and the cylinder walls, and the mass leakage are predicted independently with correlations available in the literature and simplified one-dimensional models, respectively. However, the turbulence induced by the mass intake through the piston rings can affect the gas- to-wall heat transfer. In order to address this dependency two complementary approaches are used. Compression and expansion processes are simulated in a gas spring configuration (i.e. without valve systems) using a computational fluid dynamics (CFD) model developed using the open-source code OpenFOAM, where the loss mechanisms are solved directly. The results are then compared with predictions from the heuristic lumped model based on heat transfer correlations. Finally, the lumped model is used to derive performance maps for a reciprocating-piston expander over a range of pressure ratios and mass flow rates.

Reciprocating-piston devices can be used as high-efficiency compressors and/or expanders. With an optimal valve design and by carefully adjusting valve timing, pressure losses during intake and exhaust can be largely reduced. The main loss mechanism in reciprocating devices is then the thermal irreversibility due to the unsteady heat transfer between the compressed/expanded gas and the surrounding cylinder walls. In this paper, pressure, volume and temperature measurements in a piston-cylinder crankshaft driven gas spring are compared to numerical results. The experimental apparatus experiences mass leakage while the CFD code predicts heat transfer in an ideal closed gas spring. Comparison of experimental and numerical results allows one to better understand the loss mechanisms in play. Heat and mass losses in the experiment are decoupled and the system losses are calculated over a range of frequencies. As expected, compression and expansion approach adiabatic processes for higher frequencies, resulting in higher efficiency. The objective of this study is to observe and explain the discrepancies obtained between the computational and experimental results and to propose further steps to improve the analysis of the loss mechanisms.

The efficiency of expanders is of prime importance for various clean energy technologies. Once mechanical losses (e.g. through valves) are minimized, losses due to unsteady heat exchange between the working fluid and the solid walls of the containing device can become the dominant loss mechanism. In this device, gas spring devices are investigated numerically in order to focus explicitly on the thermodynamic losses that arise due to this unsteady heat transfer. The specific aim of this study is to investigate the behaviour of real gases in gas springs and compare this to that of ideal gases in order to attain a better understanding of the impact of real gas effects on the thermally losses in reciprocating piston expanders and compressors. A CFD-model of a gas spring is developed in OpenFOAM. Three different gas models are compared: an ideal gas model with constant thermodynamic and transport properties; an ideal gas model with temperature-dependent properties; and a real gas model using the Peng-Robinson equation of state with temperature and pressuredependent properties. Results indicate that, for simple, monoand diatomic gases like helium or nitrogen, there is a negligible difference in the pressure and temperature oscillations over a cycle between the ideal and real gas models. However, when looking at a heavier (organic) molecule such as propane, the ideal gas model tends to overestimate the temperature and pressure compared to the real gas model, especially if no temperature dependency of thermodynamic properties is taken into account. Additionally, the ideal gas model (both alternatives) underestimates the thermally induced loss compared to the real gas model for heavier gases. Real gas effects must be taken into account in order to predict accurately the thermally induced loss when using heavy molecules in such devices.

The versatile precursors [Ru(CH-CHC6H4Me-4)Cl(CO)(BTD)(PPh3)2] (BTD = 2,1,3-benzothiadiazole) and [Ru(C(C=CPh)-CHPh)Cl(CO)(PPh3)2] were treated with isonicotinic acid, 4-cyanobenzoic acid, and 4-(4-pyridyl)benzoic acid under basic conditions to yield [Ru(vinyl)(O2CC5H4N)(CO)(PPh3)2], [Ru(vinyl)(O2CC6H4CN-4)(CO)(PPh3)2], and [Ru(vinyl){O2CC6H4(C5H4N)-4}(CO)(PPh3)2], respectively. The osmium analogue [Os(CH-CHC6H4Me-4)(O2CC5H4N)(CO)(PPh3)2] was also prepared. cis-[RuCl2(dppm)2] was used to prepare the cationic compounds [Ru(O2CC5H4N)(dppm)2](+) and [Ru{O2CC6H4(C5H4N)-4}(dppm)2](+). The treatment of 2 equiv of [Ru(C(C=CPh)-CHPh)(O2CC5H4N)(CO)(PPh3)2] and [Ru(O2CC5H4N)(dppm)2](+) with AgOTf led to the trimetallic compounds [{Ru(C(C=CPh)-CHPh)(CO)(PPh3)2(O2CC5H4N)}2Ag](+) and [{Ru(dppm)2(O2CC5H4N)}2Ag](3+). In a similar manner, the reaction of [Ru(O2CC5H4N)(dppm)2](+) with PdCl2 or K2PtCl4 yielded [{Ru(dppm)2(O2CC5H4N)}2MCl2](2+) (M = Pd, Pt). The reaction of [RuHCl(CO)(BTD)(PPh3)2] with HC=CC6H4F-4 provided [Ru(CH-CHC6H4F-4)Cl(CO)(BTD)(PPh3)2], which was treated with isonicotinic acid and base to yield [Ru(CH-CHC6H4F-4)(O2CC5H4N)(CO)(PPh3)2]. The addition of [Au(C6F5)(tht)] (tht = tetrahydrothiophene) resulted in the formation of [Ru(CH-CHC6H4F-4){O2CC5H4N(AuC6F5)}(CO)(PPh3)2]. Similarly, [Ru(vinyl)(O2CC6H4CN-4)(CO)(PPh3)2] reacted with [Au(C6F5)(tht)] to provide [Ru(vinyl){O2CC6H4(CNAuC6F5)-4}(CO)(PPh3)2]. The reaction of 4-cyanobenzoic acid with [Au(C6F5)(tht)] yielded [Au(C6F5)(NCC6H4CO2H-4)]. This compound was used to prepare [Ru(CH-CHC6H4F-4){O2CC6H4(CNAuC6F5)-4}(CO)(PPh3)2], which was also formed on treatment of [Ru(CH-CHC6H4F-4)(O2CC6H4CN-4)(CO)(PPh3)2] with [Au(C6F5)(tht)]. The known compound [RhCl2(NC5H4CO2)(NC5H4CO2Na)3] and the new complex [RhCl2{NC5H4(C6H4CO2)-4}{NC5H4(C6H4CO2Na)-4}3] were prepared from RhCl3·3H2O and isonicotinic acid or 4-(4-pyridyl)benzoic acid, respectively. The former was treated with [Ru(CH-CHC6H4Me-4)Cl(CO)(BTD)(PPh3)2] to yield [RhCl2{NC5H4CO2(Ru(CH-CHC6H4Me-4)(CO)(PPh3)2}4]Cl. As an alternative route to pentametallic compounds, the Pd-coordinated porphyrin [(Pd-TPP)(p-CO2H)4] was treated with 4 equiv of [Ru(CH-CHR)Cl(CO)(BTD)(PPh3)2] in the presence of a base to yield [(Pd-TPP){p-CO2Ru(CH-CHR)(CO)(PPh3)2}4] (R = C6H4Me-4, CPh2OH). Where R = CPh2OH, treatment with HBF4 led to the formation of [(Pd-TPP){p-CO2Ru(-CHCH-CPh2)(CO)(PPh3)2}4](BF4)4. [(Pd-TPP){p-CO2Ru(dppm)2}4](PF6)4 was prepared from [(Pd-TPP)(p-CO2H)4] and cis-[RuCl2(dppm)2]. The reaction of AgNO3 with sodium borohydride in the presence of [Ru(O2CC5H4N)(dppm)2](+) or [RuR{O2CC6H4(C5H4N)-4}(dppm)2](+) provided silver nanoparticles Ag@[NC5H4CO2Ru(dppm)2](+) and Ag@[NC5H4{C6H4CO2Ru(dppm)2}-4](+).

The paper presents an analysis of the effects of irreversibility on the performance of a reverse Joule–Brayton cycle heat pump for domestic heating applications. Both the simple and recuperated (regenerative) cycle are considered at a variety of operating conditions corresponding to traditional (radiator) heating systems and low-temperature underfloor heating. For conditions representative of typical central heating in the UK, the simple cycle has a low work ratio and so very high compression and expansion efficiencies and low pressure losses are required to obtain a worthwhile COP. An approximate analysis suggests that these low loss levels would not necessarily be impossible to achieve, but further investigation is required, particularly regarding irreversible heat transfer to and from cylinder walls. In principle, recuperation improves the cycle work ratio, thereby making it less susceptible to losses, but in practice this advantage is compromised when realistic values of recuperator effectiveness are considered.

Purpose: To analyze and compare retreatment rates after wavefront-optimized photorefractive keratectomy (PRK) and LASIK and determine risk factors for retreatment. Methods: A retrospective chart review was performed to identify patients undergoing PRK or LASIK with the wavefront-optimized WaveLight platform from January 2005 through December 2006 targeted for a plano outcome and to determine the rate and risk factors for retreatment surgery in this population. Results: Eight hundred fifty-five eyes were analyzed, including 70 (8.2%) eyes with hyperopic refractions and 785 (91.8%) eyes with myopic refractions. After initial treatment, 72% of eyes were 20/20 or better and 99.5% were 20/40 or better. To improve uncorrected visual acuity, 54 (6.3%) eyes had retreatments performed. No significant differences in retreatment rates were noted based on age ( P =.15), sex ( P =.8), eye ( P =.3), PRK versus LASIK ( P =1.0), room temperature ( P =.1) or humidity ( P =.9), and no correlation between retreatment rate and month or season of primary surgery ( P =.4). There was no correlation between degree of myopia and retreatment rate. Eyes were significantly more likely to undergo retreatment if they were hyperopic (12.8% vs 6.0%, P =.006) or had astigmatism ⩾1.00 diopter (D) (9.1% vs 5.3%, P =.04). Conclusions: Retreatment rate was 6.3% with the WaveLight ALLEGRETTO WAVE excimer laser. This rate was not influenced by age, sex, corneal characteristics, or environmental factors. Eyes with hyperopic refractions or astigmatism ⩾1.00 D were more likely to undergo retreatment. [ J Refract Surg . 2009;25:273–276.]