Chemical Time Scale Research Articles

Analysis of the Eddy Dissipation Concept formulation for MILD combustion modelling

Performance of the Eddy Dissipation Concept (EDC) in the regime of Moderate and Intense Low-oxygen Dilution (MILD) combustion is investigated. The special MILD features, where chemical and turbulence time scales are comparable (Damköhler number close to unity), have led several researchers to suggest modifications of EDC, mainly by changing model constants. EDC with standard and modified constants are compared, and the importance of each effect is outlined. Different fine-structure reactor models and their inflow/initial conditions are discussed and found to play a significant role. The reacting fraction of fine structures, which in virtually all other numerical studies is set to unity, is also discussed and found to be important. We observe better agreement with experiment when the reacting fraction is reduced below unity, which is also described by the original EDC. The results obtained with the variable reacting fraction are found to improve both the temperature distributions and the lift-off height predictions. The calculations are carried out with the use of open source software OpenFOAM. The main test case was the Delft Jet-in-Hot-Coflow burner emulating MILD regime at three different flow conditions (jet Reynolds numbers of 2500, 4100 and 8800).

Stabilization and blowout characteristics of lean premixed turbulent flames behind a backward-facing step in a rectangular combustor with heated propane-air mixtures

The flame stabilization and blowout velocity of lean premixed turbulent flames in a backward-facing step combustor were investigated experimentally with well-defined stable flames. The dynamic pressure in the combustor was monitored to select a stable regime among overall flame behaviors. The responses in the flame behaviors to changes in the initial conditions of equivalence ratio, inlet temperature, and Reynolds number were simultaneously captured using high-speed CH∗ chemiluminescence. The results showed that the flame modes were categorized into four representative regimes according to their dynamic behavior, overall sound pressure level, and frequency spectrum: (1) stable regime, (2) unstable regime, (3) quasi-unstable regime, and (4) transition regime. For lean premixed turbulent flames in the stable regime, the flame distance, defined as distance x from the step to flame appearance, increased with the decrease in the initial equivalence ratio, and blowout occurred near the lower flammability limits. The blowout velocities linearly correlated with both the inverse of chemical reaction time (α0/SL,02)−1 and extinction strain rate εext,0 under different initial conditions. The two chemical time scales were obtained using different flame models based on either the premixed laminar flame-speed or premixed opposed-flow flame model. In addition, the flame distances showed a linear correlation of df ∝ U0/SL,02 for all the data in the stable regime. This result implies that the lean premixed turbulent flames could be stabilized at flame distances where the local mean flow velocity is balanced with the turbulent flame speed, which is proportional to the square of laminar burning velocity.

On the force–velocity relationship of a bundle of rigid bio-filaments

In various cellular processes, bio-filaments like F-actin and F-tubulin are able to exploit chemical energy associated with polymerization to perform mechanical work against an obstacle loaded with an external force. The force–velocity relationship quantitatively summarizes the nature of this process. By a stochastic dynamical model, we give, together with the evolution of a staggered bundle of Nf rigid living filaments facing a loaded wall, the corresponding force–velocity relationship. We compute the evolution of the model in the infinite wall diffusion limit and in supercritical conditions (monomer density reduced by critical density ρ^1>1), and we show that this solution remains valid for moderate non-zero values of the ratio between the wall diffusion and the chemical time scales. We consider two classical protocols: the bundle is opposed either to a constant load or to an optical trap setup, characterized by a harmonic restoring force. The constant load case leads, for each F value, to a stationary velocity Vstat(F;Nf,ρ^1) after a relaxation with characteristic time τmicro(F). When the bundle (initially taken as an assembly of filament seeds) is subjected to a harmonic restoring force (optical trap load), the bundle elongates and the load increases up to stalling over a characteristic time τOT. Extracted from this single experiment, the force–velocity VOT(F;Nf,ρ^1) curve is found to coincide with Vstat(F;Nf,ρ^1), except at low loads. We show that this result follows from the adiabatic separation between τmicro and τOT, i.e., τmicro ≪ τOT.

Recent Trends in the Production, Combustion and Modeling of Furan-Based Fuels

There is growing interest in the use of furans, a class of alternative fuels derived from biomass, as transportation fuels. This paper reviews recent progress in the characterization of its combustion properties. It reviews their production processes, theoretical kinetic explorations and fundamental combustion properties. The theoretical efforts are focused on the mechanistic pathways for furan decomposition and oxidation, as well as the development of detailed chemical kinetic models. The experiments reviewed are mostly concerned with the temporal evolutions of homogeneous reactors and the propagation of laminar flames. The main thrust in homogeneous reactors is to determine global chemical time scales such as ignition delay times. Some studies have adopted a comparative approach to bring out reactivity differences. Chemical kinetic models with varying degrees of predictive success have been established. Experiments have revealed the relative behavior of their combustion. The growing body of literature in this area of combustion chemistry of alternative fuels shows a great potential for these fuels in terms of sustainable production and engine performance. However, these studies raise further questions regarding the chemical interactions of furans with other hydrocarbons. There are also open questions about the toxicity of the byproducts of combustion.

A new EDC approach for modeling turbulence/chemistry interaction of the gas-phase of biomass combustion

Small-scale grate-firing biomass combustors generate high level of pollutants as a result of poor air/fuel mixing and short residence time of combustion. Reliable gas-phase combustion modeling can play a key role in the improvement and optimization of the design of a biomass furnace. This paper proposes a new combustion model based on the framework of the well-known eddy dissipation concept (EDC) approach. The ability of EDC to incorporate detailed chemical reactions in turbulent reacting flows has made it attractive for modeling combustion. However, its application poses a challenge especially for modeling weakly turbulent reacting flow conditions, as well as reacting flows with comparable flow and chemical time scales. The newly proposed combustion scheme can reasonably describe the interaction of chemistry and turbulent flow over a wider range of turbulence conditions. To validate the model, simulations are carried out using diffusion and partially premixed jet flames covering weakly to highly turbulent flow conditions. Moreover, the capability of the new model in predicting NOx emissions, based on fuel bound nitrogen source as well as thermal NOx, is investigated. All numerical results are compared with their counterparts’ experimental measurements of temperature and species concentrations. Compared to the standard EDC, the results of the new EDC approach reveal noticeable improvement in the predictions, especially for slow-forming and kinetically dominated species such as CO and NOx.

Measurement and Characterization of Hydrogen-Deuterium Exchange Chemistry Using Relaxation Dispersion NMR Spectroscopy.

One-dimensional heteronuclear relaxation dispersion NMR spectroscopy at 13C natural abundance successfully characterized the dynamics of the hydrogen-deuterium exchange reaction occurring at the Nε position in l-arginine by monitoring Cδ in varying amounts of D2O. A small equilibrium isotope effect was observed and quantified, corresponding to ΔG = -0.14 kcal mol-1. A bimolecular rate constant of kD = 5.1 × 109 s-1 M-1 was determined from the pH*-dependence of kex (where pH* is the direct electrode reading of pH in 10% D2O and kex is the nuclear spin exchange rate constant), consistent with diffusion-controlled kinetics. The measurement of ΔG serves to bridge the millisecond time scale lifetimes of the detectable positively charged arginine species with the nanosecond time scale lifetime of the nonobservable low-populated neutral arginine intermediate species, thus allowing for characterization of the equilibrium lifetimes of the various arginine species in solution as a function of fractional solvent deuterium content. Despite the system being in fast exchange on the chemical shift time scale, the magnitude of the secondary isotope shift due to the exchange reaction at Nε was accurately measured to be 0.12 ppm directly from curve-fitting D2O-dependent dispersion data collected at a single static field strength. These results indicate that relaxation dispersion NMR spectroscopy is a robust and general method for studying base-catalyzed hydrogen-deuterium exchange chemistry at equilibrium.

Seasonal stratospheric photochemistry on Uranus and Neptune

A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales. At high altitudes, hydrocarbon mixing ratios are typically largest where the solar insolation is the greatest, leading to strong hemispheric dichotomies between the summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures and deeper, slower chemistry and diffusion lead to latitude variations that become more symmetric about the equator. On Uranus, the stagnant, poorly mixed stratosphere confines methane and its photochemical products to higher pressures, where chemistry and diffusion time scales remain large. Seasonal variations in hydrocarbons are therefore predicted to be more muted on Uranus, despite the planet’s very large obliquity. Radiative-transfer simulations demonstrate that latitude variations in hydrocarbons on both planets are potentially observable with future JWST mid-infrared spectral imaging. Our seasonal model predictions for Neptune compare well with retrieved C2H2 and C2H6 abundances from spatially resolved ground-based observations (no such observations currently exist for Uranus), suggesting that stratospheric circulation — which was not included in these models — may have little influence on the large-scale meridional hydrocarbon distributions on Neptune, unlike the situation on Jupiter and Saturn.

Comparing laser induced plasmas formed in diode and excimer pumped alkali lasers.

Lasing on the D1 transition (62P1/2 → 62S1/2) of cesium can be reached in both diode and excimer pumped alkali lasers. The first uses D2 transition (62S1/2 → 62P3/2) for pumping, whereas the second is pumped by photoexcitation of ground state Cs-Ar collisional pairs and subsequent dissociation of diatomic, electronically-excited CsAr molecules (excimers). Despite lasing on the same D1 transition, differences in pumping schemes enables chemical pathways and characteristic timescales unique for each system. We investigate unavoidable plasma formation during operation of both systems side by side in Ar/C2H6/Cs.

Assessment of Chemical Time Scale for a Turbine Burner

Understanding the relationship between chemical kinetics and turbulent mixing characteristic time scales provides the necessary knowledge to establish the type of combustion regime, which, furthermore, helps grasping the main characteristics of the flame physics. One of the most convenient tools for evaluating the turbulence/chemistry interaction is provided by the analysis of the Damköhler number field distribution. Literature survey provides several examples of Damköhler number computation, mostly regarding single global chemical reaction. The present work aims at applying this procedure for the calculation of the chemical time-scale (and thus of the Damköhler number) for the two-step kinetic mechanism of Westbrook and Dryer used in the simulation of gas reheat inside a turbine stage. Based on this methodology, we may be able to establish which chemistry-turbulence interaction model is more appropriate for a specific simulation (finite-rate eddy dissipation concept, flamelet approach, etc.). The research has set forth important variations of the Damköhler number (in the order of hundreds of units) throughout the flow domain. In the wake of the injector where the turbulent intensity is high and the incipient incomplete mixing prohibits fuel ignition, the flow is completely controlled by turbulence and Damköhler number is low. Hence, the appropriate turbulence/chemistry model could very well be an eddy dissipation concept type. Similar approach should be chosen for the rotor-stator passage, also. Conversely, in the rotor passage, due to the blocking effect of the walls and the strong interactions between fluid tensions and pressure, the turbulence intensity is diminished and the appropriate fuel oxidizer mixing triggers ignition and flame sustainability.

The effect of turbulence on mass transfer rates of small inertial particles with surface reactions

The effect of turbulence on the mass transfer between a fluid and embedded small heavy inertial particles that experience surface reactions is studied. For simplicity, the surface reaction, which takes place when a gas phase reactant is converted to a gas phase product at the external surface of the particles, is unimolar and isothermal. Two effects are identified. The first effect is due to the relative velocity between the fluid and the particles, and a model for the relative velocity is presented. The second effect is due to the clustering of particles, where the mass transfer rate is inhibited due to the rapid depletion of the consumed species inside the dense particle clusters. This last effect is relevant for large Damköhler numbers, where the Damköhler number is defined as the ratio of the turbulent and chemical time scales, and it may totally control the mass transfer rate for Damköhler numbers larger than unity. A model that describes how this effect should be incorporated into existing simulation tools that utilize the Reynolds averaged Navier–Stokes approach is presented.

How does the groundwater influence the water balance of a lowland lake? A field study from Lake Stechlin, north-eastern Germany

The subsurface catchment of Lake Stechlin is quite complex. Water level measurements in groundwater wells with different screen depths show that groundwater in the lake’s catchment may partly discharge to the lake and partly leak to deeper aquifers. Simple steady-state 2D vertical modelling of eight different scenarios (different leakage areas, different hydraulic conductivities) was conducted in order to explore the local and regional groundwater flow system around Lake Stechlin and the origin and possible magnitude of lacustrine groundwater discharge. A comparison of the different scenarios with in situ measurements of lacustrine groundwater discharge by temperature sticks and seepage meters revealed that only a part of the groundwater recharge in the catchment discharges to the lake while the rest leaks into deeper aquifers. This implies a differentiation of the subsurface catchment into a local and a regional flow system and impacts on travel times and chemical reaction time scales.

Finite-rate chemistry modelling of non-conventional combustion regimes

The present work deals with the numerical simulation of Adelaide Jet in Hot Co-flow burner under Moderate or Intense Low oxygen Dilution (MILD) conditions, using the Partially Stirred Reactor (PaSR) model for turbulence-chemistry interactions. The PaSR model assumes that reactions are confined in specific regions of the computational cells, whose mass fractions depend both on the mixing and the chemical time scales. Therefore, the appropriate choice of mixing and chemical time scales becomes crucial to ensure the predictivity of the numerical simulations. Results show that the most appropriate choice for the evaluation of the mixing time scale in MILD regime consists in using the geometric mean of the Kolmogorov and the integral mixing time, rather than Kolmogorov or integral scales. This is supported by the validation of the numerical results against experimental profiles of temperature, major and minor species mass fractions. Moreover, the identification of the key species in the oxidation process allows to determine a chemical time scale that improves the prediction of centerline and upstream temperature predictions, compared to the results obtained by including all species in the evaluation of the chemical scale.

Lean methane flame stability in a premixed generic swirl burner: Isothermal flow and atmospheric combustion characterization

Gas turbine combustors operating in lean premixed mode are known to be susceptible to flame blowoff due to competing influences of increasing chemical timescales and decreasing flow time scales under these conditions. In this study, combustion stability and the onset of flame blowoff in particular, are characterized in a new swirl burner operated with fully premixed methane (CH4) and air at thermal power of 55 kW, atmospheric combustor inlet pressure, and ambient (∼290 K) combustor inlet temperature. The onset of flame blowoff was shown repeatedly to exhibit high amplitude, low frequency combustion instabilities as a result of periodic flame extinction and reignition events. In addition to detailed isothermal characterization of the burner velocity field using particle image velocimetry, a combination of dynamic pressure sensing and optical combustion diagnostics, including OH∗ chemiluminescence and OH planar laser induced fluorescence, give indication of the combustion rig acoustic response and changes in flame acoustic response, heat release, and flame anchoring location related to the onset and occurrence of blowoff. This analysis shows that the onset of this instability was preceded by a marked reduction in dominant frequency and amplitude until frequency collapse and high amplitudes were observed throughout the burner inlet mixing plenum, burner pilot, combustion chamber, and exhaust ducting. Acoustic and optical signal analysis show potential viability for use in practical applications for precursor indications of lean blowoff. The flame anchoring location within the combustion chamber was shown to detach from the burner exit nozzle and stabilize within the outer and central recirculation zones near the lean blowoff limit, providing evidence of changes to both chemical and flow time scales. Chemical kinetic modelling is used in support of the empirical studies, in particular highlighting the relationship between maximum heat release rate and OH∗ chemiluminescence intensity.

3D direct numerical simulations of reactive mass transfer from deformable single bubbles: An analysis of mass transfer coefficients and reaction selectivities

The influence of the bubble diameter on the mass transfer coefficient and the reaction selectivity of a competitive-consecutive mechanism has been investigated by means of 3D Direct Numerical Simulations (DNS) of reactive mass transfer from deformable single bubbles. In order to assess the sensitivity of the transfer processes with respect to the chemical time scales, different chemical reaction intensities have been considered, covering the range from slow to fast reactions. Simulations have been performed using an Arbitrary Lagrangian Eulerian (ALE) Interface-Tracking algorithm, allowing for highly accurate hydrodynamic and reactive mass transfer results.With the concentration boundary layers being significantly thinner than the hydrodynamic boundary layer at the bubble interfaces, a two-mesh approach with a specialised refinement strategy has been adopted. In this approach, the reactive processes are numerically solved by using a second mesh with additional cell layers at the interface, alleviating the multiscale problem but keeping the computational costs reasonable.In the bubble diameter range investigated in this work, significant changes of the mass transfer coefficient and the reaction selectivity have been observed and discussed. Furthermore, the simulation results have been used to assess the validity of the film theory, also for technically relevant bubble sizes and reaction conditions.

Effects of Turbulence on Ignition of Methane–Air and n-Heptane–Air Fully Premixed Mixtures

ABSTRACTWe performed direct numerical simulations (DNS) for the two-dimensional (2D) turbulent ignition of ultra-lean methane–air and n-heptane–air mixtures with a high exhaust gas recirculation (EGR) rate at high pressure to determine the ignition criteria and ignition delay time. We defined an initial high-temperature region as an ignition kernel and conducted one-dimensional preliminary DNS to determine the ignition criteria in terms of the ignition source energy and the thermal conduction from the ignition kernel during the induction period. Additionally, we analyzed the 2D DNS results to clarify the influence of the turbulent strain rate on the ignition delay time and the mechanism by which the turbulence influences the establishment of the ignition kernel. We observed that the distribution of eddies and the strain rate in the high-temperature region influences the success or failure of the ignition process and, therefore, the ignition delay time. The ignition delay time increases proportionally to the square of strain rate averaged in the high concentration region of the intermediate species during the induction period. This suggests that the ignition in a turbulent field is based on the balance between the influence of a locally averaged strain rate in the preheating region and the chemical (flame) time scale. Based on these observations, a simple model for the ignition delay time was constructed based on the mean strain rate in the high concentration region of the intermediate species during the induction period. The strain rate averaged in the high concentration region of the intermediate species was normalized by using the laminar burning velocity and the laminar thermal flame thickness. Additionally, the ignition delay time was normalized by the ignition delay time of the corresponding laminar case, yielding the same ignition model/criterion for both examined fuels, which could be extended to other mixtures.

Model form uncertainty quantification in turbulent combustion simulations: Peer models

Turbulent combustion simulations invoke a number of component models for chemical kinetics, turbulence, flame structure, etc., each of which has an error associated with its structural form and contributes to overall uncertainties in simulation results. These model form errors arise from the necessity of making assumptions in deriving a model. Conventional approaches to estimating model form errors rely on an ad hoc additive error that is then calibrated against experimental or computational data. These approaches inherently neglect any a priori knowledge of physics in developing the model error estimate. Instead, in this work, an inherently physics-based approach to estimating model form error is developed based on the notion of “peer” models. In the generic approach, the error in a candidate model is determined by taking the difference between it and an equally plausible alternative “peer” model with a different set of assumptions. The generic approach is applied in this work to the modeling of the subfilter mixture fraction dissipation rate, which is typically modeled as the ratio of the subfilter mixture fraction variance and a time scale. The typical time scale approximation invokes a turbulent time scale, and a “peer” model is proposed in which a chemical time scale is invoked to estimate the model form error. Using stochastic collocation, the subfilter mixture fraction dissipation rate model form error as well as the uncertainty in a model parameter are propagated through LES calculations of the Sandia D Flame utilizing the steady flamelet model. The results indicate that the mixture fraction, temperature, and carbon monoxide uncertainties increase with downstream distance due to an increase in the relative subfilter mixture fraction variance and increased sensitivity to the time scale approximations, which diverge in magnitude with downstream distance. Uncertainties in these quantities arising from the model form error are shown to be more significant than uncertainties arising from the model constant uncertainty. For the temperature, uncertainties due to chemical kinetic rate uncertainty are shown to be slightly smaller than uncertainties due to the subfilter mixture fraction dissipation rate model error; for the carbon monoxide mass fraction, uncertainties due to chemical kinetic rate uncertainty are twice as large as uncertainties due to the subfilter mixture fraction dissipation rate error since carbon monoxide is more kinetically-controlled than the temperature.

Effect of improved accelerating method on efficient chemistry calculations in diesel engine

With detailed chemical kinetics being employed in combustion simulations, its major computational challenge is the time-intensive nature of chemical kinetics integration due to the large number of chemical species and wide range of chemical timescales involved. In this work, an extended tabulated dynamic chemistry approach with dynamic pruning method is carried out to simulate complex spray combustion for non-premixed combustion process. The thought of extended tabulated dynamic chemistry approach with dynamic pruning is achieved by selecting the optimum acceleration method as well as its error tolerances at different combustion stages depending on combustion characteristics involving the low-temperature combustion. The present method is applied to realistically complex combustion systems involving spray flame of n-heptane fuel and non-premixed combustion engine. Computation efficiency of the proposed method is compared with the results using different accelerating methods, including dynamical adaptive chemistry, in situ adaptive tabulation, and coupled method of tabulated dynamical adaptive chemistry. The results show that transient computational cost will decrease for low-temperature combustion by reducing ambient oxygen concentration clearly in spray flame. Meanwhile, very low computational efficiency is presented once the autoignition occurs, especially at the initial oxygen concentration of 21%. Based on the feature, extended tabulated dynamic chemistry approach with dynamic pruning with different dynamic adaptive chemistry error tolerances is proposed to improve computational efficiency. Extended tabulated dynamic chemistry approach with dynamic pruning with larger error tolerance [Formula: see text] improves around two times for decreased amplitude of transient computational cost at high-temperature combustion stage, and at the same time, the computational accuracy is also improved by comparing the important intermediate species obtained by direct integration. For applications in diesel engine, the results show that extended tabulated dynamic chemistry approach with dynamic pruning can accurately capture the first-stage ignition feature that determines the high-temperature combustion stage. In addition, extended tabulated dynamic chemistry approach with dynamic pruning with the smaller in situ adaptive tabulation error tolerance of 0.001 only used at the high-temperature combustion stage significantly improves the performance on diesel engine simulation with a larger chemistry mechanism. The present method further significantly improves computational efficiency with an overall speedup factor of 10 with high-accuracy compared with result using direct integration.

Correlation effects between turbulence and the conversion rate of pulverized char particles

The effect of turbulence on heterogeneous reactions on the surface of char particles embedded in a turbulent oxidizer, consisting of oxygen and carbon-dioxide, is in this work studied numerically. It is shown that for a small Damköhler number (Da), which is the ratio between a turbulent and a chemical time scale, the char conversion rates are somewhat increased by the turbulence. This is found to be due to the increased mass transfer rate to the char particle surface that is caused by the turbulence-induced relative velocity between the char and the oxidizer. For large Damköhler numbers, however, the char conversion rate is strongly reduced due to particle clustering. This reduction is explained by the fact that when particles are clustered in densely populated particle clusters, the transfer of oxygen to the particles in the centre of the clusters is reduced since the oxygen is consumed by the particles closer to the external surface of the cluster. At the same time, high concentrations of oxygen exist in the voids between the particle clusters. This oxygen cannot take part in the conversion of the char until it is transported to the char surface. The effects of turbulence on the heterogeneous reaction rates are furthermore modelled based on Direct Numerical Simulation (DNS) data for a simplified reacting gas particle system.

Flame stabilization mechanism study in a hydrogen-fueled model supersonic combustor under different air inflow conditions

Flame stabilization mechanisms of a hydrogen-fueled laboratory model scramjet combustor are numerically studied. The combustor consists of a sonic fuel jet injected paralleling into a supersonic air flow at the base of a wedge-shaped strut. The numerical calculations are based on an Open Source Field Operation and Manipulation (OpenFOAM) solver using the large eddy simulation method. A skeletal mechanism for hydrogen-air chemistry including 9 species 27 reactions is used to depict the combustion dynamics. Three cases with air stagnation temperature of 568 K, 460 K, and 960 K, are respectively studied under the same equivalence ratio and entrance Mach number. Damköler number based on the subgrid turbulent mixing time scale and the instantaneous chemical time scale is used to distinguish the role of mixture self-igniting from flame propagation in different air inflows. Flame stabilization in such strut-based combustor is compared with that of cavity-based one. It is found that the basic working principles for the two types of combustor are similar. However, the strut shows more simplicity in flame stabilization locations.

Synthesis and incorporation of 13C-labeled DNA building blocks to probe structural dynamics of DNA by NMR.

We report the synthesis of atom-specifically 13C-modified building blocks that can be incorporated into DNA via solid phase synthesis to facilitate investigations on structural and dynamic features via NMR spectroscopy. In detail, 6-13C-modified pyrimidine and 8-13C purine DNA phosphoramidites were synthesized and incorporated into a polypurine tract DNA/RNA hybrid duplex to showcase the facile resonance assignment using site-specific labeling. We also addressed micro- to millisecond dynamics in the mini-cTAR DNA. This DNA is involved in the HIV replication cycle and our data points toward an exchange process in the lower stem of the hairpin that is up-regulated in the presence of the HIV-1 nucleocapsid protein 7. As another example, we picked a G-quadruplex that was earlier shown to exist in two folds. Using site-specific 8-13C-2′deoxyguanosine labeling we were able to verify the slow exchange between the two forms on the chemical shift time scale. In a real-time NMR experiment the re-equilibration of the fold distribution after a T-jump could be monitored yielding a rate of 0.012 min−1. Finally, we used 13C-ZZ-exchange spectroscopy to characterize the kinetics between two stacked X-conformers of a Holliday junction mimic. At 25°C, the refolding process was found to occur at a forward rate constant of 3.1 s−1 and with a backward rate constant of 10.6 s−1.