Multi-species Transport Model Research Articles

Modelling of the groundwater flow and of tracer movement in the porous and fissured media: Chalk Aquifer (Northern part of Paris Basin, France)

A groundwater flow model has been developed in order to study the chalk aquifer of Paris Basin, based on most of the geological and hydrological available data. The numerical processes are intended to modelling the groundwater flow in the Senonian (Late Cretaceous) formations and to visualize the tracer movement in groundwater resources in the experimental site of LaSalle Beauvais (northern part Paris Basin). Both objectives were achieved as follows: (i) the comprehension of the spatial distribution of the hydraulic conductivity in the chalk aquifer taking into account the characteristics of the hydrogeological system and (ii) the use of the analytical solution for describing one-dimensional to two-dimensional solute transport in a unidirectional steady-state flow tracer with scale-dependent dispersion. Advection and diffusion mechanisms are taken into account. Comparison between the breakthrough curves of the analytical and the numerical solutions provided an excellent agreement for various ranges of scale-related transport parameters of interest. The developed power series solution facilitates fast prediction of the breakthrough curves at each observation point. Thus, the derived new solutions are widely applicable and are very useful for the validation of numerical transport. The numerical approach is carried out by MT3DMS, a Modular 3-D Multi-Species Transport Model for Simulation of Advection, Dispersion, and Chemical Reactions of Contaminants in Groundwater Systems, and based on total variation-diminishing method using the ULTIMATE algorithm. The estimation of the infected surface could constitute an approach in water management and allows to prevent the risks of pollution and to manage the groundwater resource from a durable development perspective. Copyright © 2015 John Wiley & Sons, Ltd.

CFD Modeling to Investigate the Performance of PEM Fuel Cells Affected By Cathodic Pressure

The proton exchange membrane (PEM) fuel cell is a promising energy conversion device. However, the cost and durability of a PEM fuel cell are major challenges needed to be overcome before it will be widely used. The simulation is crucial for the design and optimization of practical devices based on deep understanding of coupled transport phenomena inside the fuel cell. It is widely known that the slower reaction in the cathode is a limiting factor and the cathodic operating pressure has a strong influence on the cell performance. In this paper, an open-source code OpenFOAM is employed to simulate the transport phenomena and electrochemical reactions in PEM fuel cells. The basic governing equations for continuity, momentum, mass and energy transfer are solved with appropriate source terms using a computational fluid dynamics (CFD) method. A steady state and three-dimensional (3D) model is applied for single flow channels and multi-functional layers, i.e., bipolar plates, gas diffusion layers (GDLs), catalyst layers (CLs) and electrolyte. The distributions of gas compositions, pressure, temperature, activation loss, cell voltage and current density are captured and presented. The O2 partial pressure on the reaction sites resulting from transport and inlet pressure suggests an influence on the cell performance. The influence of removal of the inert gas N2 and production of H2O vapor on the O2 partial pressure is considered based on the multi-species transport model. To verify the accuracy of the simulation approach, comparisons between the computed results and literature data have been conducted.

Assessment of contaminant migration in an unconfined aquifer around an open dumping yard: Perungudi a case study

The study assesses the leachate migration into the groundwater using a modular finite-difference groundwater flow model and modular 3D multi-species transport model. Perungudi dumpsite in the southern part of Chennai Metropolitan Area has been taken to study the contaminant migration. The slug test conducted in the wells show that the hydraulic conductivity K is 7.98 × 10−3 cm/sec in sandy clay and 3.8 × 10−3 cm/sec for weathered rocky zone. Four-layered 3D model is formulated using the lithological data obtained from various sources. The groundwater flow model has been calibrated and validated for transient condition. By taking chloride as an index of contamination, the modelling results reveal that the contaminant may be transported for a distance of 4.5 km in 12 years (2008–2020) under the prevailing condition along the south eastern direction. If the dumping increases by twice, the contaminant concentration will get doubled in a period 10 years and if the recharge increases by 10 %, the contaminant concentration gets diluted from 600 to 100 mg/l which may be due to recharge from the marsh which favours higher dilution.

Application of three different methods to evaluate the nitrate pollution of groundwater in the Arborea plain (Sardinia – Italy)

This paper describes three different methods used to evaluate the nitrate contamination from agricultural practices in a study area located in the Nitrate Vulnerable Zone (NVZ) of the Arborea plain (Sardinia - Italy). Potential risk of contamination and concentration of nitrate pollution in groundwater has been estimated by using Parametric, Numerical and Artificial Neural Networks methods. Parametric methods consider the combination of intrinsic aquifer vulnerability to contamination index (SINTACS) and agricultural nitrates hazard index (IPNOA). The transport numerical model is based on flow model, obtained with modular three dimensional finite difference groundwater flow model (MODFLOW), and it is made applying 3-D Multi-Species Transport Model (MT3D). Artificial Neural Networks (ANNs) are used for the estimation of the nitrate concentration in monitoring well.

Transport and biodegradation modeling of gasoline spills in soil–aquifer system

The leakage from the oil pipeline and storage tank could get into soil and groundwater and lead to contamination. An understanding of the fate of gasoline spills in soil and groundwater is necessary for designing better monitoring systems and remediation strategies. In this paper, the numerical simulation method for the transport and biodegradation of gasoline spills was built by combining Hydrocarbon Spill Screening Model (HSSM) in vadose zone and modified MT3DMS (a Modular Three-Dimensional Multispecies Transport Model) in saturated zone. First, sensitivity analysis of the required time for gasoline through vadose zone was performed. Then, a three-dimensional soil–aquifer system contaminated by a gasoline spill was simulated, the vertical migration and dissolution of gasoline in vadose zone was analyzed by HSSM, and the transport and biodegradation of dissolved benzene in groundwater was computed by modified MT3DMS. The results demonstrate that the transport and biodegradation of gasoline in soil–aquifer system could be simulated by combining HSSM and modified MT3DMS. The sensitivity analysis shows that (1) if the leakage rate is much higher, the required time of gasoline through vadose zone is much shorter, and that (2) if water saturation is much higher, the required time of gasoline through vadose zone is also much shorter. The results for three-dimensional soil–aquifer system show that: (1) Vertical migration of gasoline takes 10.9 days to pass through the 5-m-high vadose zone; (2) the area of oil lens on the groundwater table enlarges about 31 times than that of source in land surface; (3) there are 94.7 % of the total mass of gasoline spills intercepted by vadose zone, and 27 % of the total mass of benzene enters and pollutes more groundwater; and (4) the microbes could grow and propagate rapidly at the place where dissolved benzene and DO (dissolved oxygen) are enough, and the contaminant plumes of dissolved benzene focus on the vicinity of groundwater table.

Heat and mass transport during a groundwater replenishment trial in a highly heterogeneous aquifer

AbstractChanges in subsurface temperature distribution resulting from the injection of fluids into aquifers may impact physiochemical and microbial processes as well as basin resource management strategies. We have completed a 2 year field trial in a hydrogeologically and geochemically heterogeneous aquifer below Perth, Western Australia in which highly treated wastewater was injected for large‐scale groundwater replenishment. During the trial, chloride and temperature data were collected from conventional monitoring wells and by time‐lapse temperature logging. We used a joint inversion of these solute tracer and temperature data to parameterize a numerical flow and multispecies transport model and to analyze the solute and heat propagation characteristics that prevailed during the trial. The simulation results illustrate that while solute transport is largely confined to the most permeable lithological units, heat transport was also affected by heat exchange with lithological units that have a much lower hydraulic conductivity. Heat transfer by heat conduction was found to significantly influence the complex temporal and spatial temperature distribution, especially with growing radial distance and in aquifer sequences with a heterogeneous hydraulic conductivity distribution. We attempted to estimate spatially varying thermal transport parameters during the data inversion to illustrate the anticipated correlations of these parameters with lithological heterogeneities, but estimates could not be uniquely determined on the basis of the collected data.

MT3DMSP – A parallelized version of the MT3DMS code

A parallelized version of the 3-D multi-species transport model MT3DMS was developed and tested. Specifically, the open multiprocessing (OpenMP) was utilized for communication between the processors. MT3DMS emulates the solute transport by dividing the calculation into the flow and transport steps. In this article, a new preconditioner, derived from Symmetric Successive Over Relaxation (SSOR) was added into the generalized conjugate gradient solver. This preconditioner is well suited and appropriate for the parallel architecture. A case study in the test field at TU Bergakademie Freiberg was used to produce the results and analyze the code performance. It was observed that most of running time would be required for the advection, dispersion. As a result, the parallel version decreases significantly running time of solute transport modeling. In addition, this work provides a first attempt to demonstrate the capability and versatility of MT3DMS5P to simulate the solute transport in fractured gneiss rock.

Simulating reactive transport of selenium coupled with nitrogen in a regional-scale irrigated groundwater system

Summary The contamination or deficiency of Selenium (Se) has become a critical issue in watershed systems worldwide during the past half-century, with either elevated or deficient Se concentrations in groundwater, surface water, soils, and associated vegetation. Regardless of the reason for concern, there is a basic need for tools for use in assessing baseline concentrations and mass loadings in large-scale environmental systems and for exploring remediation strategies. This paper presents the application of UZF-RT3D, a multi-species variably-saturated reactive transport model for groundwater systems, and its recently developed Se agricultural and chemical reaction module to a regional study site (50,600 ha) along the irrigated Arkansas River Valley in southeastern Colorado. The study region has been monitored for Se contamination during the previous decade, with all segments of the Arkansas River impaired with respect to Se. The Se reaction module accounts for near-surface Se cycling due to agricultural practices, oxidation–reduction reactions, and sorption, and includes a nitrogen cycle and reaction module due to the dependence of Se transformation and speciation on the presence of nitrate (NO 3 ). Of prime importance is the role NO 3 plays in oxidation of residual Se from marine shale, which is prevalent throughout the study region, along with its inhibition of Se chemical reduction to less toxic forms. The model is corroborated against groundwater solute concentrations, mass loadings of solutes to the Arkansas River, relationships between solutes within the groundwater system, and overall regional statistics of groundwater solute concentration. Results indicate that the model is successful in replicating the major spatial and temporal patterns in Se and NO 3 contamination and transport, hence providing a tool that can be used with confidence to explore remediation schemes.

Modeling hydrology, groundwater recharge and non-point nitrate loadings in the Himalayan Upper Yamuna basin

The mountainous Himalayan watersheds are important hydrologic systems responsible for much of the water supply in the Indian sub-continent. These watersheds are increasingly facing anthropogenic and climate-related pressures that impact spatial and temporal distribution of water availability. This study evaluates temporal and spatial distribution of water availability including groundwater recharge and quality (non-point nitrate loadings) for a Himalayan watershed, namely, the Upper Yamuna watershed (part of the Ganga River basin). The watershed has an area of 11 600km2 with elevation ranging from 6300 to 600m above mean sea level. Soil and Water Assessment Tool (SWAT), a physically-based, time-continuous model, has been used to simulate the land phase of the hydrological cycle, to obtain streamflows, groundwater recharge, and nitrate (NO3) load distributions in various components of runoff. The hydrological SWAT model is integrated with the MODular finite difference groundwater FLOW model (MODFLOW), and Modular 3-Dimensional Multi-Species Transport model (MT3DMS), to obtain groundwater flow and NO3 transport. Validation of various modules of this integrated model has been done for sub-basins of the Upper Yamuna watershed. Results on surface runoff and groundwater levels obtained as outputs from simulation show a good comparison with the observed streamflows and groundwater levels (Nash–Sutcliffe and R2 correlations greater than +0.7). Nitrate loading obtained after nitrification, denitrification, and NO3 removal from unsaturated and shallow aquifer zones is combined with groundwater recharge. Results for nitrate modeling in groundwater aquifers are compared with observed NO3 concentration and are found to be in good agreement. The study further evaluates the sensitivity of water availability to climate change. Simulations have been made with the weather inputs of climate change scenarios of A2, B2, and A1B for end of the century. Water yield estimates under climate change scenarios have been made and implications on groundwater and groundwater quality have been assessed. The delicate groundwater resource balance that connects livelihoods of millions of people seems to be under tremendously increasing pressure due to the dynamic conditions of the natural environment of the region and the future climate changes.

Numerical modeling and simulation of condensation heat transfer of a flue gas in a bundle of transport membrane tubes

In this paper, a numerical study has been carried out to investigate the heat and mass transfer characteristics of a condensing combustion flue gas in a cross-flow transport membrane tube bundle. The tube wall is made of a porous material that is able to extract condensate liquid from the flue gas. The flue gas investigated consists of one condensable water vapor (H2O) and three noncondensable gases (CO2, O2, and N2). A simplified multi-species transport model was developed for the heat and mass transfer of flue gas. The condensation-evaporation process was simulated as a two-step chemical reaction. The RNG two-equation turbulence model was used for the turbulent flow. The numerical study was conducted within ranges of Reynolds number of 1.0×103–7×104 based on hydraulic diameter of flue gas channel, and 6.4×100–3.3×102 based on inner diameter of the water tube. Flue gas inlet temperature is within the range of 333.2–360.9K. Numerical results were compared with our experimental data. It has been found that the developed multi-species transport model was able to predict the flue gas heat and mass transfer in the tube bundle with fairly good accuracy. The heat and mass depletion levels decrease with the increase of the flue gas Reynolds numbers. A new Nusselt number correlation was developed for flue gas convection in the tube bundle. Detailed results about temperature, mass fraction, enthalpy, and skin fraction factors are also presented and discussed.

Regimes of chemical reaction waves initiated by nonuniform initial conditions for detailed chemical reaction models

Regimes of chemical reaction wave propagation initiated by initial temperature nonuniformity in gaseous mixtures, whose chemistry is governed by chain-branching kinetics, are studied using a multispecies transport model and a detailed chemical model. Possible regimes of reaction wave propagation are identified for stoichiometric hydrogen-oxygen and hydrogen-air mixtures in a wide range of initial pressures and temperature levels, depending on the initial non-uniformity steepness. The limits of the regimes of reaction wave propagation depend upon the values of the spontaneous wave speed and the characteristic velocities of the problem. It is shown that one-step kinetics cannot reproduce either quantitative neither qualitative features of the ignition process in real gaseous mixtures because the difference between the induction time and the time when the exothermic reaction begins significantly affects the ignition, evolution, and coupling of the spontaneous reaction wave and the pressure wave, especially at lower temperatures. We show that all the regimes initiated by the temperature gradient occur for much shallower temperature gradients than predicted by a one-step model. The difference is very large for lower initial pressures and for slowly reacting mixtures. In this way the paper provides an answer to questions, important in practice, about the ignition energy, its distribution, and the scale of the initial nonuniformity required for ignition in one or another regime of combustion wave propagation.

Modeling the transport and retention of nC60 nanoparticles in the subsurface under different release scenarios

The escalating production and consumption of engineered nanomaterials may lead to their increased release into groundwater. A number of studies have revealed the potential human health effects and aquatic toxicity of nanomaterials. Understanding the fate and transport of engineered nanomaterials is very important for evaluating their potential risks to human and ecological health. While there has been a great deal of research effort focused on the potential risks of nanomaterials, a limited amount of work has evaluated the transport of engineered nanomaterials under different release scenarios in a typical layered geological field setting. In this work, we simulated the transport of fullerene aggregates (nC60), a widely used engineered nanomaterial, in a multi-dimensional environment. A Modular Three-Dimensional Multispecies Transport Model (MT3DMS) was modified to evaluate the transport and retention of nC60 nanoparticles. Hypothetical scenarios for the introduction of nanomaterials into the subsurface environment were investigated, including the release from an injection well and the release from a waste site. Under the conditions evaluated, the mobility of nC60 nanoparticles was found to be very sensitive to the release scenario, release concentration, aggregate size, collision efficiency factor, and dispersivity of the nanomaterial.

MT3DMS: Model Use, Calibration, and Validation

MT3DMS is a three-dimensional multi-species solute transport model for solving advection, dispersion, and chemical reactions of contaminants in saturated groundwater flow systems. MT3DMS interfaces directly with the U.S. Geological Survey finite-difference groundwater flow model MODFLOW for the flow solution and supports the hydrologic and discretization features of MODFLOW. MT3DMS contains multiple transport solution techniques in one code, which can often be important, including in model calibration. Since its first release in 1990 as MT3D for single-species mass transport modeling, MT3DMS has been widely used in research projects and practical field applications. This article provides a brief introduction to MT3DMS and presents recommendations about calibration and validation procedures for field applications of MT3DMS. The examples presented suggest the need to consider alternative processes as models are calibrated and suggest opportunities and difficulties associated with using groundwater age in transport model calibration.

Development of a multi-species mass transport model for concrete with account to thermodynamic phase equilibriums

In this study, a coupled multi-species transport and chemical equilibrium model has been established. The model is capable of predicting time dependent variation of pore solution and solid-phase composition in concrete. Multi-species transport approaches, based on the Poisson–Nernst–Planck (PNP) theory alone, not involving chemical processes, have no real practical interest since the chemical action is very dominant for cement based materials. Coupled mass transport and chemical equilibrium models can be used to calculate the variation in pore solution and solid-phase composition when using different types of cements. For example, the physicochemical evaluation of steel corrosion initiation can be studied by calculating the molar ratio of chloride ion to hydroxide ion in the pore solution. The model can, further, for example, calculate changes of solid-phase composition caused by the penetration of seawater into the concrete cover. The mass transport part of the model is solved using a non-linear finite element approach adopting a modified Newton–Raphson technique for minimizing the residual error at each time step of the calculation. The chemical equilibrium part of the problem is solved by using the PHREEQC program. The coupling between the transport part and chemical part of the problem is tackled by using a sequential operator splitting technique and the calculation results are verified by comparing the elemental spacial distribution in concrete measured by the electron probe microanalysis (EPMA).

Evaluating MT3DMS for Heat Transport Simulation of Closed Geothermal Systems

Owing to the mathematical similarities between heat and mass transport, the multi-species transport model MT3DMS should be able to simulate heat transport if the effects of buoyancy and changes in viscosity are small. Although in several studies solute models have been successfully applied to simulate heat transport, these studies failed to provide any rigorous test of this approach. In the current study, we carefully evaluate simulations of a single borehole ground source heat pump (GSHP) system in three scenarios: a pure conduction situation, an intermediate case, and a convection-dominated case. Two evaluation approaches are employed: first, MT3DMS heat transport results are compared with analytical solutions. Second, simulations by MT3DMS, which is finite difference, are compared with those by the finite element code FEFLOW and the finite difference code SEAWAT. Both FEFLOW and SEAWAT are designed to simulate heat flow. For each comparison, the computed results are examined based on residual errors. MT3DMS and the analytical solutions compare satisfactorily. MT3DMS and SEAWAT results show very good agreement for all cases. MT3DMS and FEFLOW two-dimensional (2D) and three-dimensional (3D) results show good to very good agreement, except that in 3D there is somewhat deteriorated agreement close to the heat source where the difference in numerical methods is thought to influence the solution. The results suggest that MT3DMS can be successfully applied to simulate GSHP systems, and likely other systems with similar temperature ranges and gradients in saturated porous media.

Effects of sorption competition on caesium diffusion through compacted argillaceous rock

We carried out a small-scale laboratory diffusion experiment on a disk-like sample of Opalinus clay from the Mont Terri underground laboratory (Switzerland) using 134Cs as tracer. A through-diffusion phase was followed by an out-diffusion phase where the tracer taken up by the sample was released again. Since the tracer concentration at both boundaries was monitored, careful mass-balance considerations were feasible. A first analysis of the experimental data was done in the frame of a single-species model accounting only for transport and non-linear sorption of caesium. The model could match the data of the through-diffusion phase, however only, when strongly reducing the sorption data based on batch sorption experiments. Yet, such a procedure was in strong contradiction with sorption measurements performed on dispersed and compacted systems. In addition, predictions concerning tracer out-diffusion and mass-balance considerations clearly revealed the shortcomings of this type of model. In a second attempt we applied a multi-species transport model where now the whole water chemistry and a sorption model for caesium were considered. First, the value for the diffusion coefficient was fixed to the best-fit value of the single-species model. But again, the sorption site densities had to be reduced strongly albeit the reduction factor was smaller. Only when fixing the sorption site densities to those values of the sorption model and letting the effective diffusion coefficient D e free for the adjustment, could through-diffusion data be reasonably well fitted and out-diffusion as well as mass-balances be predicted in a satisfying manner. The main results are: (1) The best-fit could be achieved with a value for D e of 1.8 × 10 −10 m 2 s −1 which is rather high but corroborated by results of a molecular modelling study. (2) If caesium arrives in the Opalinus clay sample potassium and sodium (calcium etc.) ions are released and caesium ions are sorbed. The released cations diffuse to lower concentration regions according to their individual concentration gradients. Since locally the cation concentration for potassium, (sodium and calcium) is increased, sorption of these cations is also locally enhanced, affecting in return the sorption behaviour of migrating caesium. Consequently, the sorption process of caesium in such diffusion experiments cannot be addressed by a non-linear isotherm formalism any longer. (3) A reasonable analysis of such single tracer diffusion experiments therefore requires the combined description of transport (diffusion) and sorption of many cations and the whole complex water chemistry of the system. Thus, single-species models can only be applied with care in the considered concentration ranges.

Stochastic uncertainty analysis for solute transport in randomly heterogeneous media using a Karhunen‐Loève‐based moment equation approach

A new approach has been developed for solving solute transport problems in randomly heterogeneous media using the Karhunen‐Loève‐based moment equation (KLME) technique proposed by Zhang and Lu (2004). The KLME approach combines the Karhunen‐Loève decomposition of the underlying random conductivity field and the perturbative and polynomial expansions of dependent variables including the hydraulic head, flow velocity, dispersion coefficient, and solute concentration. The equations obtained in this approach are sequential, and their structure is formulated in the same form as the original governing equations such that any existing simulator, such as Modular Three‐Dimensional Multispecies Transport Model for Simulation of Advection, Dispersion, and Chemical Reactions of Contaminants in Groundwater Systems (MT3DMS), can be directly applied as the solver. Through a series of two‐dimensional examples, the validity of the KLME approach is evaluated against the classical Monte Carlo simulations. Results indicate that under the flow and transport conditions examined in this work, the KLME approach provides an accurate representation of the mean concentration. For the concentration variance, the accuracy of the KLME approach is good when the conductivity variance is 0.5. As the conductivity variance increases up to 1.0, the mismatch on the concentration variance becomes large, although the mean concentration can still be accurately reproduced by the KLME approach. Our results also indicate that when the conductivity variance is relatively large, neglecting the effects of the cross terms between velocity fluctuations and local dispersivities, as done in some previous studies, can produce noticeable errors, and a rigorous treatment of the dispersion terms becomes more appropriate.

A linearization technique for multi-species transport problems

We study a one-dimensional multi-species system of dispersive-advective contaminant transport equations coupled by nonlinear biological (kinetic reactions) and physical (adsorption) processes. To deal with the nonlinearities and the coupling, and to avoid additional computational costs, we propose a linearization technique based on first-order Taylor’s series expansions. A stabilized finite element in space, combined with an Euler implicit finite difference discretization in time, is used to approximate the dispersive-advective transport problem. Three computational tests are performed with different boundary conditions, retardation factors and kinetic parameters for a nonlinear reactive multi-species transport model. The proposed methodology is shown to be accurate and decrease computational costs in the numerical implementation of nonlinear reactive transport problems.

Detailed analysis of the mass burning rate of stretched flames including preferential diffusion effects

This study focuses on the effects of flame stretch on the mass burning rate, for flames with nonunit Lewis numbers. The extended flame stretch model of de Goey and ten Thije Boonkkamp [Combust. Flame 119 (1999) 253–271], which was derived for multiple-species transport and chemistry, is used as a starting point. This model is adjusted to be able to predict the mass burning rate at the inner reaction layer of the flame. The accuracy of the theory is analyzed step by step using numerical results incorporating detailed chemistry and multispecies transport models. The adjusted model proves to be an accurate predictor of the mass burning rate. Furthermore, it is shown that not only the lean species Lewis number plays a role but also a number of other Lewis numbers have a significant influence on preferential diffusion. Preferential diffusion for methane/air mixtures is more difficult to predict accurately than for ethane/air and propane/air mixtures. Different contributions to the total preferential diffusion effect in methane/air mixtures partly cancel, which corresponds to an effective Lewis number being close to one. For fuels with Lewis numbers further away from one, no cancelation takes place and the accuracy of the predicted mass burning rate increases. Results show that the extended theory forms the basis for the first quantitative model to predict mass burning rates of premixed, laminar, stretched flames with nonunit Lewis numbers.

Temperature profile measurements in the near-substrate region of low-pressure diamond-forming flames

Coherent anti-Stokes Raman scattering (CARS) spectroscopy of diatomic hydrogen was used to measure gas-phase temperature profiles in low-pressure, stagnation-flow, diamond-forming flames. The objectives of the study were to measure detailed temperature profiles for comparison with a numerical flame model and to investigate the presence and magnitude of the temperature jump at the deposition substrate surface. At distances of several hundred microns or greater from the deposition substrate, the measured temperatures were in excellent agreement with the results of a numerical flame model incorporating elementary chemical kinetic and multi-species transport models. The estimated hydrogen CARS temperature accuracy was ±4% near the deposition substrate and was ±5% away from the substrate. The estimated spatial resolution of the measurements was 30 μm normal to the surface, and the gradient in temperature was resolved fully up to approximately 25 μm from the surface. The excellent spatial resolution for the near-surface measurements allowed the investigation of the presence and the magnitude of the discontinuity between the gas and surface temperature at the deposition substrate, a phenomenon known as the temperature jump. Temperature jumps of approximately 100 K were observed in these rich, premixed oxy-acetylene flames ranging from 30 Torr. to 125 Torr. The influence of gas flow rate and pressure on the temperature jump was investigated. The observed temperature jumps are consistent with an accommodation co-efficient in the range of 0.1 to 0.2 for the exchange of energy between the diamond-forming flame gases and the molybdenum surface or the diamond film on the deposition substrate. In these low-pressure flames, the ability to fully resolve the near-substrate temperature profiles will be extremely useful for the validation and improvement of surface chemistry models.